One of the earliest examples of this is Space Invaders, where the very last enemy ship suddenly gains an enormous speed boost and descends upon the player in mere seconds! Apparently this was originally an Ascended Glitch.
In Pac-Man, Blinky is already red, and already slightly faster than the other monsters. When you get close to clearing a level, he gets faster, and pursues you even when the scatter timer is on. He does this in sequels, too, at least up through Pac-Mania (1988).
The Wigglers in Super Mario World turn red when you stomp on them once. Their face turns from happy to angry, their walking speed increases, and they chase the player.
Bowser's clown car also does this in the Final Boss battle: the eyes narrow and it starts slamming the ground.
Enemies in Bubble Bobble, Symphony/II, and Memories will turn red when they are the last on the screen. Their movement speed increases. They also Turn Red when time starts to run out in a level, or if you trap them in a bubble and don't burst it in time. It gets very annoying when Super Drunk does this...
Ditto with Mario Bros., except that the last enemy on the level turned blue. Thus, killing a blue enemy completes the level. This also happens if you leave an enemy stunned long enough for it to wake up... except they turn green and get even faster when this happens. This also happens more directly with the Sidesteppers, who get angry and move faster the first time they're bumped.
Given a Shout-Out in Golden Sun: Turtles and crabs moving around the field in a coin-tossing game would progressively Turn Red (in the same colors, even) and move faster if hit by the coin.
Most of the enemies in God Hand, including the bosses, go all red in the face and become more aggressive if you hit them hard enough. Or if you taunt them. Why would you want to taunt them? Because it helps build the Limit Break bar (and the Dynamic Difficulty).
The Legend of Dragoon color-codes the player's targeting icon according to an enemy's remaining HP; every enemy in the game will unveil at least one new attack when it's reduced to half its maximum HP (when the targeting marker turns yellow).
In Shin Megami Tensei III: Nocturne and Digital Devil Saga, some enemies have a type of move that effectively gives them two to four free actions. Random encounter enemies will often use it when they are the last one remaining, and bosses use it at certain points to power up and kick your ass. That being said, not all enemies have the courtesy to wait turning red before using that kind of move. Oh, and don't think that they'll be courteous and use the moves only once per attack phase. There are times where they will attack until they have one turn left, get more moves and repeat. You can predict what happens because of this.
Ratton viruses in Mega Man Battle Network became angry if struck once and not killed, increasing their movement speed and dodging rate and firing off two homing bombs instead of just one. They didn't turn red, but they put on little angry eyes. If you happened to hit them again, they'd become nigh-impossible to track, fire faster, and in some cases fire off their bombs in groups of three. Of course, they also had significantly less HP than their peers in the area, so it was mainly just to punish careless players.
While more minibosses than anything else, Darknuts in The Legend of Zelda games routinely do this. It's somewhat justified because the player's attacks at first only damages their armor. When they lose it, they get faster. In The Legend of Zelda: Twilight Princess, they also throw their heavy sword or mace at Link and then draw a rapier, adjusting their combat tactic to a more agile approach.
In Devil May Cry, on the hardest difficulty level, every enemy can Devil Trigger if not killed fast enough. This makes them tougher, faster, much more aggressive and sometimes grants them new moves. Not only that, but in 3 they will also devil trigger if you kill two other enemies while they're present, on the highest difficulty level, that is. On easier difficulty levels they'd just get scared. ( Which is just as awesome. )
Lots of melee enemies in World of Warcraft are prone to this. Most of them will actually turn red and grow larger when it happens. The effect is generally called "Enrage", "Frenzy", "Berserk" and other similar words, and generally has the effect of increasing the enemy's damage and attack speed. Some variations of it are more sophisticated: some Ogres, for instance, will have their damage increased by a LOT, but their attack and movement severely decreased. Caster mobs generally don't turn red. Bosses have more complicated examples, see below.
In Chrono Trigger, the Gold Eaglets in 65,000,000 BC do this literally. There's also Mother Brain, who will go berserk if you destroy her healing monitors. She gradually gains boosts to her attack and defense, meaning she'll become literally impossible to kill if you let the battle drag on too long. These aren't the only examples, of course, but they're the most notable.
In one of the last updates to the roguelikeADOM, most opponents gained the ability to occasionally "fight to the death in a blind rage". A bug in the first iteration of this trait sometimes made them commit suicide instead.
In Monster Hunter, most large monsters have a "rage" mode that they go into when they receive a certain amount of damage, which increases their speed and damage output and occasionally makes them start using new attacks. However, unlike most of the examples on this page, the monsters in Monster Hunter will eventually calm down over time. A couple of bosses actually become reckless when they get enraged, leaving them vulnerable to abilities which otherwise don't work on them (Though the majority of them become immune or resistant to the effects of traps and flash/sonic bombs). Certain monsters like the Gigginox and Deviljho also change their weak spots while they're mad.
Trouble is, the more health they lose, the easier it is to knock them back into rage mode. (i.e. the monster goes into rage the first time after 5 minutes of a beating and calms down. He goes into rage a second time after only 2 minutes.) This continues until a single hit will put them into rage mode. This is usually a sign that the monster is about to die.
One mission in the Challenge Tower mode of Mortal Kombat 9 has the player, as Noob Saibot, fighting a version of Smoke whose uniform and hair periodically become red. When that happens, Smoke gains a status buff, doing extra damage for about five seconds. This is handwaved by the game as an effect of Shao Kahn's brainwashing.
The Berserker enemies in Ninja Gaiden Black were white, skeletal knights carrying a BFS around like it was nothing, and were generally Demonic Spiders normally. When pissed off, they turn black, and ramp up their ridiculousness even further. Definitely a Boss in Mook Clothing.
Also, Ogres were freaky monsters wearing triceratops skulls as masks. Break 'em, and they flip the hell out. Like the berserkers, they were tough as nails before flipping out, and only get tougher.
In Plants vs. Zombies, the newspaper zombies will get angry and start to move faster once you've shot at them enough to knock the newspaper out of their hands.
Dark Chronicle/Dark Cloud 2 has this with the "Rage counters". On each enemy, there is a series of red tokens over it's health bar, and it's a different amount for each enemy (usually, the lower the count, the lower the relative defense). Every time you hit it or, if you are using a machine gun, shoot it a certain number of times (it's somewhere between 5 and 10), one of the counters goes away. If all the counters go away, then you better start fighting smarter or run, cause its attack level will effectively QUADRUPLE!
In Kingdom Hearts II, the Hot Rod heartless will charge in a berserker rage once their HP is low. Most times they strike you from where you're not looking. (This is a real pain at higher difficulties if you're at a lower level).
The Large Body heartless similarly go berserk at low HP and start bellysliding after you at an astonishing speed, but can be stopped by blocking and/or using a reaction command on them, unlike the previous example.
The aptly named Berserker Nobodies also do this, repeatedly using an annoyingly long, unstoppable combo on you that renders them nigh impossible to hit with anything but magic while they're performing it, and only giving you a few seconds to land a combo of your own between each assault.
Some of the Dream Eaters in Kingdom Hearts 3D like the Hebby Repp, Pricklemane and Zolephant will turn bright red once they've received enough abuse, granting them an increase to their speed and attack power, along with upgrading some of their attacks. When used as allies, the appropriate disposition can make them start off every battle in this state.
The Kirby franchise has included Scarfy, an enemy that looks like a floating ball with wiggling cat ears. If you try to inhale it, it will turn and start pursuing you. Previously it was stationary, if harmful. It can be defeated only by copy abilities, sliding, or throwing something at it.
They were featured in Kirby of the Stars as well, except Kirby could use his inhale power against them.
It also has a penchant for blowing up after chasing you for a certain length of time. Whut indeed.
There's also another enemy in Kirby's Dreamland 2 and 3 that would remain dormant unless Kirby was riding on one of his friends, in which after getting too close would put up the scariest face ever and chase after Kirby. Oh, and it's a One-Hit Kill to the animal friend.
In Kirby and the Amazing Mirror there's an enemy called the Snooter that literally turns red if you don't kill it in one hit.
In Halo 3, if a Brute squad leader is killed, the Grunts sometimes turn kamikaze. Similarly, Ultras and other high-ranking Elites rush you with an energy sword when critically damaged, and Brutes go Berserk when their armor is destroyed or their leader is killed.
Super Crate Box strives to be the best example of this trope by making any enemy that reaches the bottom of the screen drop from the top with a new (red colored) sprite. You can see it in action in this video
Happens literally in Killing Floor with the Flesh Pound. All his lights turn from yellow to red when you piss it off which means his life support just started pumping him full of adrenaline and stimulants, causing him to go berserk and live up to his name.
In Serious Sam 3, major biomechanoids start to fire volley of rockets when heavily damaged.
In Space Panic, if an enemy is trapped in a hole but the player doesn't fill the hole in, the enemy will eventually climb out and turn into a stronger type (unless it already was the strongest type, of course). Ironically, the red enemy type is the weakest.
In Mr. Do!'s Castle, by the same company as Space Panic, the enemy unicorns could get stronger if hit too many times, turning from red to green to blue.
In Resident Evil 4, starting with Chapter 2, Plagas randomly emerge from Ganados whose heads are blown off, and starting in Chapter 3, they can decapitate Leon. Similarly, the Adjules in Resident Evil 5 split their heads lengthwise into Silent Hill-style vertical jaws, which can also decapitate the player characters, and the some of the Plagas that emerge from Majini can detach and fly around. Uroboros speeds up significantly and shields its weak points better; the two Exploding Barrels are best saved for this phase (if you're trying to kill it without using the furnace, so you can get its treasure).
In the remake of Resident Evil zombies that are not decapitated or set on fire come back later as Crimsons Heads, red zombies that lose the Zombie Gait and grow long claws.
Big Daddies in BioShock gain a speed boost when their HP is half-depleted.
Yetis from the Are We There Yeti?! DLC for Orcs Must Die! 2 literally turn red when brought below 50% health, switching from ranged attack to berserk charge that allows them to go right over the barricades.
Mook angels from Bayonetta can be taunted into rage state where their attacks become stronger. It also activates when you keep damaging them for a while.
Diedough-Goos from The Wonderful 101 turn red and go on a rampage if damaged enough. During this state their attacks are unblockable.
In Borderlands 2, it's possible to knock the helmet off a Goliath's head. This reveals their hideously mutated heads and angers them so much they drop their guns, regenerate all their health, and attack the first thing they see, friend or foe. They also literally turn the color red. If they kill other enemies in this state, they get bigger, stronger, and tougher, and give more XP when killed.
In Door Door, after eliminating all but one enemy on a level, the last enemy will start moving faster than the player.
The Big Boo boss from Yoshi's Island. Hit him and he inflates, making him more dangerous; hit him enough and he pops.
More or less all bosses in the Wario Land series of games do this. These bosses don't all change shape, but it's a universal rule that the closer they are to death, the more furiously they'll attack and the faster they'll move. The page picture is of the very first boss of Wario Land 4, a harmless eggplant-shaped critter who suddenly turns berserk once you've hit her enough times.
All bosses in Ys 3: The Oath in Felghana do this at half health. They gain new attacks which deal even more damage and are usually even harder to dodge than their standard attacks they begin with. Harder difficulties of the game give them another Turn Even Redder mode at 25% health left, giving them more attacks.
Bunches of bosses in the Metroid Prime series do this too. Ridley is one notorious example, as are the guardian bosses from Prime 2 (Jump Guardian, Bomb Guardian, Spider Guardian...) and the final bosses of all three games (Metroid Prime, Emperor Ing/Dark Samus, and DS/AU 313).
Many regular enemies in Prime 3 would enter Hypermode at random intervals. The bosses would also alternate hypermode after sustaining enough damage.
In Tekken 6, all characters will enter Rage mode at low health, but the boss will actually begin to glow cherry red when he's losing, and can pretty much kill you with one hit you at that point.
Speaking of Tekken, Heihachi will do this in Street Fighter X Tekken, should he activate Pandora mode. Usually, the character who you're sacrificing to get Pandora mode will crumple up on the floor and the character receiving Pandora will become a purple/black mix. However, when giving Heihachi Pandora, he simply throws his partner out of the ring and turns red.
Every boss from Final Fantasy XII will do at least one of: unlocking more powerful attacks, doubling their power and toughness (effectively meaning their lifebar is 150% long) or turning invincible(!) The only practical way to defeat some is to damage them to just above the threshold, and then unleash the characters' one-use-only ultimate combo attacks and pray. When applied to some of the game's wicked optional bosses, things just get ridiculous.
Several Marks gain the hidden ability "CT=0" which allows them to throw attacks at you ridiculously quickly, once they hit critical HP. (It sets the Charge Time of all of their actions to zero, meaning attacks are repeated just as often as their animation allows. Oh, and Haste doubles the animation framerate.)
The player can also turn red if you've unlocked certain augments, with characters having a better chance to unleash a devastating combo when on low health or suffering various status ailments.
The status "Berserk" cloaks the affected character in shadowy-red with glowing eyes (with the standard loss of direct control but greatly enhanced auto-attack damage) in a literal example of this trope in effect.
By end-game, every single enemy can Turn Red. If their hits can combo, critical health makes their combo rate skyrocket. No exceptions. Most of the beefier enemies (Bunes, Behemoths) have their attack and defense ratings jump by fifty percent. Some (Reavers, Behemoths) unlock powerful moves, and even if you kill them the attack (usually magical and area-of-effect) will still execute. Other scrappers (Abaddon, Crusader) ignore evasion while their combo rate and attack go up. Entites have, at half health, zero MP cost, and then they get more abilities when they reach critical. This all is not counting what powerups the Bonus Dungeon versions of these regular monsters get—and let's not even start with the Rare Game monsters.
Almost every single boss in Final Fantasy III DS has extra moves that aren't used until they lose a certain amount of their health.
Final Fantasy IV had the Calco and Brena dolls, which unless you manage to wipe out the last few of them at once, merge into the giant Perverse Puppet Calcobrena. Even worse, the dolls have a nasty habit of using Self Destruct when there are only a few of either kind left, which triggers the transformation into Calcabrina by reducing their numbers.
Enuo from the Gameboy Advance remake of Final Fantasy V does a somewhat subtle version of this when he reaches roughly half health; all damage dealt to him is halved and he begins using dangerous counter attacks. This isn't a visual change, but is rather indicated by the message "The power of the Void is increasing!"
In fact, most Final Fantasy bosses tend to have complex scripts that dictates the boss to do different (and usually more nasty) things at lower HP. The ones listed above are just the tip of the iceberg, really (though for most of them, the powerup aren't as obvious).
In the second Golden Sun, the Final Boss is a three-headed dragon. Whenever a head is killed, the boss gains access to more powerful attacks. On the other hand, it loses actions with each head, so that by the time it is down to the last head, it only has 2 actions (like every other boss.) It's also using both turns to Beam Spam you, so it's not like the fight is subtly getting easier.
In many games of the Shin Megami Tensei franchise, bosses tend to turn red. Some start using stronger or different moves, some call for help and so on. Persona 2's Big Bad also changes his dialogue when you talk to him in battle. If a boss starts talking in the midst of battle, it's usually a bad sign.
Persona 3: Nyx Avatar does this, first by having you fight him through several relatively weak patterns until you get to the Death arcana, when his defense and attack power go through the roof. He takes it even further when he breaks out Moonless Gown. And don't even mention Night Queen to some of the people who've beaten the game... Of course, you can just bust out Armageddon after he shifts to Death, scoring a One-Hit Kill and skipping the second half of the fight.
In the premiere gameplay video of Diablo III, the Barbarian fights the Thousand Pounder mini-boss. At half health, he turns black with glowing tattoos and becomes much faster.
In Unlimited Sa Ga, the sequentialFinal Boss loses parts as you advance through the battle, losing the attacks related to those parts and starting to use new stronger ones. It also starts to get more attacks per turn. Then, at the final stage, it loses part of its... hmm, head?, and also its weapons which were power limiters and turns bright red. He gets a HP refill as well which means you'll have to hit him a bit before he start losing more LPs again. At this point, all of his attacks are strong and/or painful, the strongest one being calledOverkill.
The final boss of SaGa II starts the battle by firing laser beams at you, upgrades to an atom smasher after taking a bit of damage, and then eventually just starts launching the atom smasher at you.
The final boss of Romancing Saga 3 puts an eclipse in the background, gains wings, and draws power from one of the four Devil Lords. If you didn't kill at least one of the four Devil Lords before this fight, you were guaranteed to lose at this point. Victory would still be possible given that you use the Game BreakerZo with Dragon God Descent and all the characters max out their WP and JP, or near maxed out.
Cerberus from Devil May Cry 3 gets bonus points for literally turning red.
Phantom from Devil May Cry turned a brighter shade of red and moved noticeably faster right around when you had him at about a fifth of his lifebar.
Nightmare chooses to Turn Red after you deplete its life bar for the last time. The formerly slow and predictable (and controllable) machine suddenly regenerates about a quarter of its health, breaking apart into a pulsing half-solid, half-goo mass that pelts Dante with lasers, missiles, and slugs while sucking away his power gauge and trying to eat him.
In fact, almost all of the Devil May Cry 3 bosses change their attack pattern or add to it after a certain amount of health is lost, usually 1/3, 1/2 and 2/3 of the life bar. Nevan's attack power is directly linked to her health, she summons more bats and lightning bolts in the same move if her health is lower. She also gets the normal pattern changes, including one that introduces a potential insta-kill life draining attack.
Most of the bosses in Devil May Cry 4 are examples of this, and do so again when playing on the hardest mode. For example Echidna the She-Viper; After hacking her lifebar down past the half-way point, not only does her attack pattern change, but the scenery turns noticeably stormier and Echidna herself starts glowing menacingly. Sanctus also visibly changes halfway through the first major battle with him when he includes the involvement of the incomplete Savior.
Devil May Cry 3's final fight against Vergil... "You're going down. AAARRGGHHHH!" Cue utter decimation of the player.
Solidus Snake from Metal Gear Solid 2. He's relatively easy at first, but once half of his health bar has been depleted, he says, "Very good, Jack, but this is where it gets interesting." He proceeds to shed his tentacles, begins spamming a lunging blade attack that leaves behind a trail of fire and is generally much more aggressive.
The Rays, once damaged enough, start using homing missiles and laser cutters.
VulcanRaven from Metal Gear Solid originally starts out stomping around the Alaskan permafrost. Shirtless. Carrying a giant vulcan cannon. After taking a bit of damage, he stops playing around and hauls ass.
Metal Gear Pythagoras of Metal Gear Ac!d literally turns red after losing both wings. Which signals the introduction of its almost-unavoidable giant laser.
Played with in Rogue Galaxy: one boss literally turns red and attacks again immediately, being twice as hard to defeat. The game even lampshades this.
"Does he think turning red is going to help?"
The challenge of the High Priestess in the Puzzle GameThe Fool's Errand comes in the form of 99 flashing buttons that need to be pressed in order. Now, you would think that this would get easier once you'd taken out enough of them that they don't overlap and jump in front of each other so often. But when the number of buttons goes below a certain number, they start jumping randomly about the screen, making this Unexpected Gameplay Change to reflex-based play that much more annoying. And when you're down to just one, it moves so fast that it is physically impossible to react to it in time, forcing you to think outside the box.
Most of the monkey bosses in Donkey Kong Jungle Beat (Except ironically the last one) trigger a cutscene at half-health of them yelling. After that point the boss gets slightly faster/harder and sometimes uses a different tactic.
Kruncha in Donkey Kong Country 2: Diddy's Kong Quest. Pretty much every boss in any of the Donkey Kong Country games adjusts attack pattern progressively harder after each hit. King Zing Sting in 2 can't be hurt while colored red.
Mega Man Powered Up gave every boss a special attack that turned the boss invincible, did huge damage unless dodged, and would be used on the player any time the boss was under half health.
In general, not too many bosses from Mega Man did this, but for the arcade games, they all did.
Starting with Mega Man 7 and the Mega Man X series, bosses reacted differently to their weakness. Some of them would appear to enter a special invulnerable animation, then unleash their special attack.
In Mega Man X3, every boss will Turn Red at 50% health, either changing the battlefield (Gravity Beetle will open a persistent black hole at the top of the room that affects jump physics and injures X if he touches it), or adding a new attack to their pattern (Tunnel Rhino gets an invincible dash attack).
The Twin Freaks fight in Dynamite Headdy has an interesting take on this. You're running through a maze, Twin Freak chasing you. Normally he's green, smiling, and relatively slow (although he does shoot projectiles). But there are switches in the maze that flip the screen. And in the flipped version of the maze, Twin Freak is red with an angry scowl, invincible, and moving much faster. You have to avoid him until you can flip the maze again and turn him back to green.
In Treasure Of The Rudras All bosses gain shades of red/dark red/purple to show how much life they have left. The darker the shade the closer they are to death.
As if it wasn't difficult enough to get and fight it, the TzTok-Jad monster in RuneScape calls a slew of healers the moment he hits half Hit Points. And if the player can't take him or them fast enough, they will heal him to max health, and reappear when he is at half HP again.
The bosses in the first Breath of Fire game will recharge a sliver of life, which takes more time and effort than the previous entire life bar to get rid of. The later games 'solved' this by hiding life bars of enemies you haven't beaten before.
Some, in particular the Grimlin and Horntoad, would "get mad" at the party and ruthlessly spam hit-all magic attacks that became harder to cope with, as there was only one cure-all spell in the game (and it was unlikely the player would have learned it at either point in the game).
There are probably a number of of MMORPG examples. In World of Warcraft, for example, most instance and raid bosses will have some variant on this, whether summoning more minions at various health levels, going berserk or pulling out an especially nasty one-time move. Most raid bosses have a time limit, after which they simply go nuts and kill everyone, no exceptions. Others get more and more powerful as time passes, eventually overwhelming their enemies (sometimes called a Soft Enrage, while the strict time-limit variant is called Hard Enrage).
The earliest one encountered is probably Herod, he is otherwise a straightforward tank and spank with occasional "avoid the whirlwind", but things gets interesting once he is brought down to 50% hit points.
An interesting variant is a boss that simply becomes immune to all damage with a shield. The shield can only be broken by luring him into special fires. Then he Turns Red.
To clarify, bosses have in the past had three different types of this effect, which used to all be called Enrage, but have since been clarified and distinguished from each other. Throughout the fight, you might see the boss Enrage ("Boss becomes enraged!"), which can be dispelled by your friendly neighborhood hunter before the boss renders the tank into fine paste or exhausts the healers. The classic Frenzy ("Boss goes into a frenzy!") happens when some bosses are at low health, and must be powered through and the fight ended quickly. Other bosses have a hard timer that cause them to Berserk ("Boss goes into a berserker rage") if you take too long, which usually makes them mash the tank for 3x his HP bar, then move onto clothies and...yeah.
Jan'alai in the 10-man Zul'Aman raid has a two-stage version of this. Five minutes into the fight, his damage and attack speed increase by 50%, making things harder but not necessarily unmanageable. Ten minutes into the fight, he goes berserk and his damage increases to the point where he can wipe the raid in seconds.
Not just that, but some bosses are specifically scripted to use other eleventh-hour abilities when their health drops to dangerously low levels. Some of these abilities are so egregious that they define the boss fight in question (e.g. summoning hordes of mooks or changing into a more badass form, requiring your party to "nuke it down" before the inevitable happens). It is genuinely rare for a WoW boss to not be at his most powerful just prior to his Critical Existence Failure. Fortunately this is generally VERY physically impressive, and makes for a very entertaining watch (Malygos blasts the chamber under you into nothingness, Kael'thas' power shatters Tempest Keep around him). Just don't get hypnotized by the awesome, or you'll end up dead.
A rare case of a boss becoming weaker later on is Magmadar in the Molten Core. Throughout the fight he repeatedly goes into a "killing frenzy" but after a certain period of time has passed, the game states "Magmadar becomes exhausted!" and his attack speed is reduced.
Warlord Kalithresh is an interesting example. He will enrage only if players fail to destroy the water tank he is channeling from in time.
Ulduar introduced another form of this, bosses that become much harder when certain conditions are fulfilled. This also grants the players better rewards... if they can handle the challenge.
Dual Boss encounters (or more) also tend to do this when one of the bosses die. The Iron Council trio gain new abilities when one of them dies, to the point that the last remaining member can be more dangerous than the original trio. Another Dual Boss, a pair of giant worms, have a similar mechanic in that both use a nasty Status Effect that can be cured by standing near a player with the effect from the other worm. The surviving worm also enrages.
Shannox is the most recent example of this. Kill one of his dogs and he enrages, gaining attack power. Kill his second dog and he does it *again*. Get his to 30% health and his dogs go berserk and 1hitko the whole raid (on normal mode, on heroic mode they somehow lose the ability to care).
Fathom-Lord Karathress follows the standard "you kill my Elite Mook, its death empowers me" model, but with a twist - if savvy players nuke the boss directly to get around this, he draws powers from the survivingElite Mooks to increase his attack speed and damage to unmanageable levels. On top of that, he berates you for trying to be cheeky.
All monsters, bosses included, in Final Fantasy XI will begin to use their special attacks more frequently (every time their TP hits 100, as opposed to randomly between 100TP and 300TP) when their HP is below 25%. Bosses frequently use their job's 2-hour ability at specific HP points, sometimes more than once or for more than one job, and may begin using more dangerous special attacks at low HP. Certain notorious monsters in particular will increase dramatically in strength and defense after a time, but this is determined by actual time elapsed and not HP loss, and is meant to make it difficult for players to hold a monster to relocate its respawn time into their own time zone. This isn't intended as difficulty increase, as more often than not they become impossible to defeat when this happens until they kill all attacking players and regenerate to full health.
A lot of bosses in City of Heroes. Every Praetorian and Arachnos Archvillain, every Freedom Phalanx Hero will, at 15% health, pop out a new fancy power, usually related to their tier 9 primary power. Most of the more memorable ones involve turning on green/rainbow/blue/red backlights and being near-invulnerable for the next three minutes. Then there are the ones who can self-resurrect and force you to defeat them again.
Bloody Stunner Chiefs. Bloody Juicer Chiefs. Bloody Tanks. Bloody Clockwork Princes. Bloody explosive enemies. (Mages, I'm looking at you. Embalmed Vahzilok zombies, I'm also looking at you.) Quite a lot of the stuff I like fighting does this...
In Rune Factory: A Fantasy Harvest Moon, the bosses literally become a reddish tint and attack faster and stronger after certain HP depletion. Especially lethal since it's really easy to forget that this happens, and it throws off the timing you've established that far. It's the same in Rune Factory 2. In Rune Factory 3, the bosses begin to flicker black and gain an extra-powerful attack.
Rune Factory Frontier takes that up a notch during post-main story rematches against dungeon bosses if they have already been beaten at least twice. They will go through the entire match in "tinted" mode and can easily curb stomp your character if unprepared.
In Castlevania: Aria of Sorrow, about halfway into the battle with Julius Belmont he unleashes his Grand Cross attack which causes towers to collapse in the background and tries to suck you into it for massive damage, and from then he on uses subweapons—though he's quite difficult already even without them, such is his badassery. but before that battle you face Balore, who midway through, uncovers his right eye and starts firing LASER VISION AT YOU, constantly!
Castlevania Order Of Ecclecia has the crab boss Brachyura that pursues you up a shaft. You need to deliberately anger it so that it will break the ceilings that are blocking your way; making it turn red is not an indication of how much health it has left (though it is an indication that you're making progress).
From the same game, Dracula himself will adopt a different strategy later on, replacing his traditional attacks with summoning wolves and bats, draining your life and actually walking after you.
Blackmore also does this. He's insanely hard already, but then he says "That was quite entertaining!" and attacks more erratically and more dangerously. In hard mode, he never says the quote and starts the fight in that mode. It's not actually that much harder as he is so difficult anyway.
Circle of the Moon has Hugh, who half way through the battle, turns on the power of evil and gets an ex skill for each of the sub weapons.
About 80 percent of the Minotaur boss fight in God of War is a simple matter of pattern recognition, blocking and successfully executing the necessary mini-games. Once you get to that final 20 percent, however, the damn beast loses its armor and starts launching unblockable super-attacks.
In Super Mario Galaxy 2, nearly as many bosses do this as well. Gobblegut actually does the whole train whistle with steam coming out of it thing, King Lakitu and Prince Pikante get visibly angry and turn red, Peewee Piranha turns red note but if you approach him from the front turns and runs away exposing his weak point, Squizzard brings out two gigantic bomb shooting cannons, Sorbetti generates red steam when annoyed and gets quicker etc.
The fourth edition of Dungeons & Dragons has the "bloodied" status applied to anyone under half health. A few creatures become stronger in this state: For instance, the Angel of Valor can turn his swords into armor-ignoring energy blades when bloodied. Conversely, however, the players can gain special abilities that make them stronger against bloodied foes, so it may be a wash.
Done a few times in Star Fox 64. The perhaps most surprising example is the robo boss at the end of Sector X, which suddenly comes back to life (even fooling the shield bar). Then again, the fact that he didn't explode like any other boss might be quite a giveaway. The second fight is however fairly easy, but you need to kill him fast or he'll hit Slippy, forcing you to play on the easy route.
"I admit defeat. IF THIS DOES NOT WORK!" (Meteor boss)
Smash TV was all about this trope, with the big boss of each stage covered in ablative amour, and progressing through several stages of aggro as the players unleashed gobs of firepower. The excesses could even be considered a parody.
This is Kracko's schtick in the Kirby franchise. You'll fight him as a weird eye thing, eventually empty his life meter... and then (mid-fight Taking You with Me fake-out optional) face a whole 'nother life meter's worth of battle against him, now with his trademark cloud and lightning attacks.
Kracko's hardly the only one. In at least three games, Whispy Woods becomes more aggressive after halving his lifebar- in one instance, he actually uproots himself and starts to chase you. Acro also does it in his two appearances in the franchise.
In Kirby's Return to Dream Land, all of the bosses get serious and start using new attacks at half health, with a change in size and/or appearance often accompanying it. Some are more notable than others. The Grand Doomer Turns Red twice: once normally, and a second time at low health that makes him Nigh Invulnerable. Only Super Abilities could damage him. The Final Boss, Magolor, Turns Red FOUR TIMES for a five phase final boss fight after the traditional Unexpected Shmup Level. However, these phases are generally divided into two forms: The first three phases that work like the Grand Doomer, then a normal boss fight for the second form. The Bonus Bosses, HR-D3 and Galacta Knight, both have very memorable Turning Red sequences. HR-D3 in the main Extra story mode is actually a giant Turning Red scenario: The Egg Engines Boss, Metal General, summons this robot after defeat in Extra Mode. HR-D3 does not have any Turn Red scenarios in its health bar. Because it partially a Call Back to Kirby 64: The Crystal Shards, meaning it has TWO WHOLE healthbars. Then, we have Galacta Knight, who is even MORE of a Bonus Boss than HR-D3, as he isn't even fought in the Extra story mode, but only in The TrueArena. He is fought EXACTLY like a normal boss, but instead of his theme from Kirby Super Star Ultra playing, he gets Landia's theme. Until he actually Turns Red. THEN his own theme music kicks in, he unleashes a combo, and starts using more deadly attacks.
I Wanna Be the Guy bosses love doing this. Kraidgief and Mecha Birdo are especially notorious for Turning Red, the former first trying to bash you and then throwing fireballs (along with a Kenshiro Spinning Piledriver if he gets too close), and then throws Blankas at you for the final phase; and the latter shooting egg platforms and Shyguys at you for the first phase, firing lasers at you for the second phase, and then going back to the Shyguy phase for the last leg, only the tips of the egg platforms in the last half kill you if you touch them.
Dracula summons two giant invincible minions that follow you around once he gets damaged enough. These minions turn red themselves and speed up.
In the final battle, The Guy grows into a huge head halfway through.
The Mega Man X series tends to like this. Starting from Mega Man X2, you can pretty much expect bosses to do something nastier/cooler if they're more than half down on their health. Possibly another upgrade when they're down to a quarter.
The Big Bad get bonus points for gradually turning red. And then, there's that boss who turns blue instead.
X8 takes this even further by giving the bosses a Desperation Attack, during which they are completely invincible.
Special notice to the true final boss who will gain a desperation attack that ends the fight in 30 seconds unless you kill him. To do so requires breaking 3 shields and removing the last of his health, no small feat ordinarily but doing it without the combo attack is truly impressive.
The trend to modify bosses' attack patterns once they're down to 1/2 or 1/4th of their hitpoints continues in the sequel series Mega Man Zero. Special mention goes to the guardians, who not only change patterns, but gain an attack that makes them invincible until they complete it. Moreover, they are covered in flame which makes it impossible to jump them without climbing a wall, which none of their areas have.
Pretty much every boss in the Mega Man Battle Network series will start attacking more often, upgrade their usual attacks, and/or start using new attacks once their health hits the 50% mark.
Zakum in MapleStory goes through this twice. He has three health bars, and his sprite looks more and more damaged as each bar is emptied. The more damaged he is, the more he uses an attack that reduces everyone's HP and MP to 1. When he's down to his last health bar, he uses the attack constantly.
Papulatus also Turns Red, although he only does it once. Although the second health bar has very little health and lacks his room-filling magic, he moves extremely fast and flies all over the room. Very deadly considering his Collision Damage is enough to kill anyone in 1 to 3 hits.
There are many bosses who do this in Final Fantasy X. There was Seymour Flux, who ushered in the devastating "Total Annihilation" attack after whittling his HP down to 35,000. There was also Braska's Final Aeon, who pulled a giant sword out of his stomach after doing enough damage.
The penultimate boss (Seymour Omnis, the form Inside Sin) takes this trope literally when it's about to cast Ultima (which could be seen as its "desperation move")
The Hanged Man becomes more evasive and starts attacking with Deadly Lunges.
The Hermit does this twice; first it retreats down the pipe and fires web balls at you, then it guards its face until it gets up in your face to attack.
The Magician employs his hard-to-counter Flash Step attack in his second and fourth phases.
In The House of the Dead 2, Hierophant starts using its jumping stab attack after losing enough life, and uses this attack exclusively when at critical health.
The Wheel of Fate in The House of the Dead 3 has three phases: one in which he randomly decides one of three attacks, one in which he throws snowball-like balls of electricity at you, and one in which his health seems to refill completely (though it goes down at the same rate as his cancel gauge) and he shoots sparks to the back of the room that reflect back and, unless shot, will hit you.
Chariot, the first boss of The House of the Dead, is a subversion. Ordinarily, he is only damageable via a chink in his armor. Hurt him enough, however, and the armor falls off... allowing the players to literally shoot him to pieces.
The Emperor turns into a floating conglomeration of spheres and his weak point become much more difficult to hit. His HOD 4 counterpart, The World, has two desperation attacks, one for each phase, both of them used only when his health is down to 15% or less. In his first form, his desperation attack consists of him creating four ice crystals into the sky, and after a time any crystals left intact mergers into a larger, sturdier, faster crystal that goes straight for your face. In his second form (where he literallyTurns Red), he creates a fast-moving ice dragon that spirals high into the sky then quickly descends and bite you. Have fun with that.
In Ristar, most bosses change color multiple times as you damage them, sometimes speeding up gradually as this happens, but the boss of Planet Undertow, a shark that you fight in a water-filled cave, features an interesting twist on this trope. Each time you damage it, it knocks out a plug in the floor of the cave and some of the water drains out, leaving you with a progressively smaller space to fight in.
In Ninja Gaiden II the giant turtles turn glowing red and constantly rain flaming rocks once they reach 1/2 hp.
A great number of bosses in Aquaria fit this trope to a T. Nautilus Prime literally turns red and gets faster; Mithala shoots bigger and faster shots; the Sun Worm moves faster and messes with the water level more; and the Sunken City boss gets really, really mad. In fact, this trope is exaggerated: when the bosses go berserk, the wholescreenturns red!
The Giant Sandworm in Tales of Hearts effectively clones itself at 1/4 health by adding its tail to the field. The tail has all the same attacks as the head and shares HP. Later in the game, another boss splits itself twice, once at half health and once at a quarter if memory serves.
In Xenosaga Episode III, as an option you could fight Omega Universitas in a simulation, he's no more powerful than he is in the mandatory confrontation meaning you could take him down in one turn, but after doing so he transforms into Omega Id and becomes easily capable of killing any one of your characters in a single attack.
Certain bosses and unique monsters in Xenoblade Chronicles will start busting out new attacks and/or activate an aura that grants them a substantial boost to their combat abilities when their health starts to dwindle. The latter can be removed by certain artes, while you just have to deal with the former.
Kingdom of Loathing: all of the bosses in the Hobopolis clan dungeon can "flip out" halfway through the fight. However, players can learn skills that let them calm a flipped out boss down.
Starting with the 6th installment of the Touhou series, most bosses will use one or more Spell Cards, usually when their current health bar is down to less than a half. (Starting with the 7th game, Perfect Cherry Blossom, the portion of the health bar where the boss will activate a Spell Card is in red.) When a boss uses a Spell Card, the background changes, and the boss's portrait is flashed on the screen. Spell Card attacks tend to be harder than regular attacks and usually make the boss's health bar drain much slower. However, defeating a Spell Card attack by draining the red portion of the boss's health bar before time runs out (or simply surviving the countdown, for Spell Cards that make the boss temporarily invincible) without getting hit by a single bullet or using a bomb awards the player a considerable score bonus.
Some bosses will also do this in the middle of a Spell Card, too. Ran Yakumo's Illusion God "Descent of Izuna Gongen" and Yukari Yakumo's Bounded Field "Boundary of Life and Death" both start out deceptively easy but fire progressively more bullets as the user's health bar decreases, and Suwako Moriya's Scourge Sign "Mishaguji-sama" decreases the pause between waves as Suwako's health bar decreases. Don't think you can just sit there without firing and time out the Spell Card to avoid the boss Turning Red, either - when the timer hits 30 seconds left, the Spell Card automatically goes to its hardest difficulty regardless of remaining health.
This is actually a common trait of the last spellcard for extra bosses, which get harder as their HP lowers by either doing it more strongly or in Mokou's case, adding more patterns to avoid. Utsuho also pulls this off, despite just being a final boss.
Jason in the NES game Friday the 13th does this. You have to defeat him several times over the course of three days/nights. In the second day he alternates between moving normal and fast, and does a bit more damage than in day one. In the third day he stays fast the entire time and does even more damage than he did on day two.
Baal from Disgaea already has utterly ridiculous stats. However, if you kill off all the mooks in his stage before him, his stats (except for HP) double.
Fraxy allows an event to check the HP of the weakpoint. If HP of weakpoint is less than 50%, play (Insert all the nasty attacks here.)
Some designers make the boss Turn Red more than once, leading to a Sequential Boss (sort of: the boss's multiple forms all have the same health bar). Often involves One-Winged Angel as well.
And now, there's two new types of bosses that turn red: "Hard Mode / What If?" Bosses and "Death Label" bosses. Hard mode bosses have secret forms/attack patterns that activate after the player does something, such as not dying or shooting the boss's tail. "Death Label" bosses are just the regular boss, just faster, deadlier, and maybe possessing more forms.
Bonus points for the fact that the health meters turn to a garish hot pink when it turns red.
Also, in the first Klonoa game, Door To Phantomile (and its Wiimake), the third boss, Gelg Bolm, Turns Red when you damage it enough. It becomes a lot faster and aggressive in this state, and the weapon it sends after you (a giant peach... thing) is a lot bigger than the first one it sends out earlier in the fight.
The Crimson Heads in the Resident Evil remake result when a Not Quite Dead zombie gets back up. They become much faster, stronger, and... red. Hence the name. Can be averted by blowing their heads off or by immolating them. The terror factor of the Crimson Heads will lead to an obsessive compulsion to ensure all zombies either die from headshots or are immolated afterwards.
The last boss of Dragon Quest IV/Dragon Warrior IV, Necrosaro, does this seven times during the battle. In the original NES version, it's the only enemy sprite in the entire game with animation. After you destroy its arms and head, another face emerges from its torso. As you damage it further, it sprouts new arms, bigger legs, and to top it all off, a second head with three eyes. In the Playstation/DS remake, it has extremely detailed animations for each transformation.
Hwa Jai from the original Fatal Fury literally turns red by chugging down a drink after losing over a quarter of his health. When juiced, he gets faster, stronger, and gains a nasty flying knee move known as the "Rocket."
Xenogears has Grahf doing this for the enemies. He will turn up, boost the current boss' power to oblivion and leaves, to the point where his entrance line seriously annoys or freaks out the player. "Does thou desire the power?"
In Mario & Luigi: Superstar Saga the last time Popple attacks you he teams up with Birdo, if you take Popple out first, Birdo loses it and says "You may have defeated my darling but that is as far as you go!" then literally turns red and begins firing off her exploding eggs in a more random pattern .
Most opponents in Punch-Out!! for the NES, and all of them for the Wii version, get more difficult the more times you knock them down.
Don't Star Punch Soda Popinski unless it's to finish him off. Especially if you have low health.
He also does it when he gets up after being knocked down, as well as if you knock the soda bottle out of his hands.
Bald Bull turns red before unleashing a Bull Charge in the Wii version.
The second time you fight him in the NES version, regular punches will not knock him down. Once his energy is low enough, you can only drop him with an uppercut or by punching him during a Bull Charge. If you dodge a charge, he'll keep doing them until one of you knocks the other down.
During the fight with Belius in Talesof Vesperia, she duplicates herself after taking a good chunk of damage, making an already hard boss battle into a controller-smasher. You can at least get rid of the Shadow version of the boss by relighting all the torches in the arena.
Practically all of the bosses in the game and throughout the series in general start using new attacks as their HP lowers. They also have an unpredictable version of this in the games where they can activate Overlimit, as it temporarily lets them spam their attacks (On top of sometimes enabling the use of new ones), makes them Immune to Flinching, and permits the usage of their Mystic Arte if they have one.
Every boss in La-Mulana does this. Even Amphisbaena becomes faster once you've racked up some damage on him.
The last GUILT, Savato, that you have to fight in Trauma Center: Under the Knife does this after you've separated it once. However, given that the game forces you into The Healing Touch at this point, this could possibly be a subversion.
Also, in Trauma Center: Under the Knife 2, Pempti, the mutated version, has two cores, destroying one quite literally causes the other one to turn red and go berserk with relentless attacks on the patient until it, or the patient, is dead. As well as this, Aletheia, the final boss, has one last desperate move when you're almost done, blood vessels around the core flash red at a rate faster than the eye can keep up with, if you haven't got the Healing Touch ready to slow things down to a regular version of cut-all-8-blood-vessels-while-they-are-not-red, then you are screwed.
Ace suits in Dynasty Warriors: Gundam will receive a reddish/pinkish/purplish glow (depending on the state) glow when they go into rage and receive quite a power up when doing so. Also your own suit will start radiating when your SP gauge is full.
Dynasty Warriors and Samurai Warriors will sometimes have enemy generals, usually ones with unique models (IE, playable characters or at least important NPCs). When knocked down or in a defensive position, they may glow for a moment and suddenly buff themselves. They will also often glow for a few seconds before unleashing a Musou attack.
Subverted and lampshaded in Terranigma. After fighting a Humongous Mecha, and getting him down to a single hit, he launches a special attack. This attack is easy to dodge, and barely hurts at all if you are hit. After launching it (and dying), the robot says "that last attack was not very impressive."
Gaoh, the boss of Samurai Shodown V, transforms into a hulking armored beast when his POW gauge fills. In this form, he is invincible and spams a lunging headbutt attack until the meter drains and he changes back.
While quite a few bosses in the MOTHER series do this, Lil Miss Marshmallow from MOTHER 3 literally goes into a Pissy Boss Mode when you do enough damage to her, changing appearance from mildly agitated to downright infuriated.
Also in Mother 3, when Boney sniffs the Steel Mechorilla, it claims that it smells "weak against thunder!" Unfortunately, using PK Thunder on the Mechorilla makes it go berserk, at which point you're screwed.
The Carbon Dog from EarthBound transforms into Diamond Dog after he has taken some damage.
Also in Kingdom Hearts II is Xaldin, who, once you've knocked off a health bar, begins to use longer combos with a larger range and less recovery time in between, spending about 90% of this time in an invincible green glow unless you use a special command on him.
Most bosses in the Kingdom Hearts series do this. They do everything from split up into multiple enemies, to cloak themselves in Darkness, to lighting on fire, to increasing their attack rate... you get the idea
The Manta Ray Boss in Super Mario Sunshine splits into smaller and faster parts when sprayed with enough water - Getting every part down to its smallest level makes the manta rays turn Red-ish Pink and chase you instead of just floating about.
Emperor Sun does a version of this in Jade Empire- however, instead of getting pissed, he gets desperate (complete with a cut scene showing his waning powers) and starts switching around his defences faster than before (ensuring the player has to change attack style with EVERY HIT in order to get around his immunities.)
Additionally, Master Smiling Hawk will magically drain the life of one of his followers when he hits half health, getting a couple of new fighting styles- and all his health back. (But he's not exactly hard to beat anyway!)
The Gigantic Burner in Parasite Eve 2 plays this straight. However, the first phase of the battle is also on a hidden timer, which can alter the fate of an NPC.
Most boss fights in A Link to the Past have multiple stages (the boss might become faster and more aggressive after taking a certain amount of damage, or Link might have to kill a group of smaller enemies before he can attack the main boss), but the one which adheres most closely to this trope is the fight against the six Armos Knightsnote fought twice, once in the Eastern Palace and once in Ganon's Tower. At first, they jump around the room in fixed patterns, but when you have killed five of them, the last one turns from blue to red and actively tries to stomp Link.
The wight-lord Vathar heals fully when almost dead, and starts spawning lots of worms.
Mordirith, who is also undead, heals fully when half dead, and announces that what you were fighting before was just an illusion, then demonstrates a few new tricks.
The Blind One starts spawning clones of itself, all at full health and hitting just as hard as the original.
In No More Heroes, every boss becomes more aggressive with depleted health. Some learn new attacks, while others employ new tactics. The first boss from the first game, for example, triplicates himself.
These "new attacks" are, in a couple of cases (Shinobu most comes to mind), unblockable insta-kill attacks which can only be avoided if the player has memorised the pose the boss takes, and knows they have to run away as fast as possible whenever they see this boss doing this.
In the arcade version of Final Fight, Rolento, the fourth boss, starts dashing around the arena rapidly and spamming grenades when his health bar gets low.
In Guild Wars Factions, Shiro Tagachi will use Meditation of the Reaper, which makes him immune to damage and lasts 30 seconds or until 500 damage has been absorbed. And when it ends, Shiro steals 20 health from each player for every second he was meditating... making it imperative to end the meditation as quickly as possible to avoid a party wipe.
Non videogame example: Dark Master Zorc does this near the end of the Monster World arc after Yugi separates Bakura from the Spirit of the Ring.
Every boss in An Untitled Story is like this. The StoneCastle face boss literally turns red as well.
Pretty much every boss in the video game adaptation of Scott Pilgrim vs. the World start flashing gold once their health has been depleted past a certain point— mimicking the players' own Hyper Mode. They then get much faster and start spamming their strongest attacks.
Many Bullet Hell shooters such as the above have evolving bosses. One unique example is ESP. Ra De (from Cave, the same company that made Do Donpachi). There were two types of bosses: mechs and ES Pers. Mechs "turned red" in pretty standard fashion like most shooters of the type. ES Pers were worse: they buffed significantly multiple times. They had multiple life gauges, and each one you emptied out turned them "redder" and "redder".
If you beat Golden Disaster with Strong Style without dying, you face Zatsuza, an even tougher literal red version of Hibachi.
The Bonus Boss of Etrian Odyssey 3: The Drowned City. It starts as a massive, screen-hogging Eldritch Abomination, which only turns out to be its bottom half. And midway in the fight against its top part, it suddenly flares up with a roar, changes shape, and starts the next phase of the fight with plenty of beatings which can easily one-shot your entire party if you're unlucky.
The Gradius games have several examples, such as Big Core MK III, which rapid-fire Beam Spams you after you destroy two of its cores, and the Berserk Core.
The Lion Keeper in Karnov. Kill the keeper first, and the lion goes berserk. Kill the lion first, and the keeper throws a stream of knives.
Most bosses in the Time Crisis series have multiple shades of red.
Several Ikaruga bosses. The third boss spins faster and fires wave motion cannons, the fourth boss also activates a laser after you destroy the hatches guarding its core, and Tageri's third form has its beam spams become faster and its Bullet Hell denser as you damage it.
Grimgrin. When you get his health down significantly, he turns orange and starts churning out projectiles like crazy, creating somewhat of a Bullet Hell situation.
When Clawbot takes enough damage, the claws first turn red and then detach from the base, which starts firing its laser more rapidly.
Many Raiden bosses. Some, such as the Base on Wheels boss in the first game, up their firepower when you destroy their optional parts. In the fourth game, several of the bosses turn red multiple times. Raiden II's fifth boss (That One Boss to many players) literally turns red in its third form.
All bosses in Ray Crisis have variable shades of red depending on the order in which the stages are played.
In Halo 2, each time you score a hit on the Heretic Leader, he retreats into the ventilation ducts, then comes back with a larger number of attack drones.
In MSF High Forum, Kim does this when she's hit in a game of dodgeball. The effect? Twice as powerful as before.
Dragon Age: Origins has numerous examples, including the Broodmother (who's a real pushover until turning red and summoning lots of mooks) and the Archdemon, who also summons mooks after getting down to about half health.
Special mention has to go to the Sloth Demon that traps your party in the Fade in the Circle Tower (Broken Circle part of the main quest). Uses an especially annoying variation of the trope as he assumes a new form when brought to near 0 HP, each with progressively deadlier abilities - and fully regenerated health. And he does this four times.
Most bosses in Blaster Master, particularly Stage 3 (speeds up) and Stage 5 (fires ever-denser bubble spams).
In [[Video Game/Rayman1995 the original game]], Mr. Dark starts turning into some of the previous bosses mashed together as he gets lower on health.
Rayman 2 has you punching bombs back at the Golgroth, the final boss. Then the roof falls in and you have to fly around shooting it into lava, whilst missiles chase you and said lava rises. Frustrating is an understatement.
In Rayman 3: Hoodlum Havoc, the final boss is incredibly hard to fight at the start. He then destroys half the arena, and things just start getting worse from there....
Sho Minamimoto does this after about 1/3 of his health is gone, gaining quicker attacks and permanently staying in his noise form for the rest of the battle.
Vespertillo Canor (the second giant bat) turns into a single golden bat similar to the one Beat and Rhyme defeated a while back. It can only attack you once on each screen for each time it pops up, and has almost no health.
White Knight Chronicles. Every boss. Every single boss has an enrage mode where they turn red and do far more damage after you dare to hit them a few times. Because this is based on how many times you damaged them and not their overall health they can enrage multiple times in one fight, making them some of the pissiest bosses in any game ever. To add to the effect many bosses will throw visible tantrums when entering enrage mode.
Fighting boss characters in Mario Party 9 uses this trope when their health gets below half. They'll either attack/move faster or have new attacks. They'll also throw a tantrum as well.
The Silent Hill series does this a number of times. For example, in Silent Hill 1, the Split-Head utilizes its One-Hit Kill vertical jaws after you blast it a few times with the shotgun, and the God in Silent Hill 3 throws faster and deadlier fire waves when its HP is low.
In Alpha Protocol, Brayko takes cocaine once you get his health down to 50% (and after that as a regular part of his attack pattern), which temporarily turns him into a knife-swinging, immune-to-stun-and-knockdown madman (well, moreso than usual) who runs supernaturally fast and will hunt you down and shank you. He turns into a subversion if you have Steven Heck on your contact list, as he will spike Brayko's drug stash which means Brayko doesn't get the speed bonuses and damages himself slightly every time he does it.
Similarly, if you kill his bodyguards Conrad Marburg will flip out and hunt you down for a good round of fistcuffs until you damage him sufficiently again. Despite being a 50-year old man using his bare hands, he's almost as nasty as Brayko in melee.
In P.N.03, Loewenzahn partially transforms from a insectoid tank into a pterosaur-on-wheels when down to about 1/3 HP, and goes fully One-Winged Angel as a robotic phoenix for the last part of the rematch battle. This last form is a pushover, however. Alraune starts shooting paralyzing homing plasma bolts at 1/2 HP, and employs its death laserBeam Spam when it's down to 1/4 HP.
In Gods Eater Burst, Hannibal (and its more powerful variant Corrosive Hannibal) gains access to a new attack where it levitates and hits you repeatedly with pillars of fire when you break the scale on its back. Much truer to this trope, the Arda Nova (and its stronger variant) becomes much faster and gains access to a completely different moveset if (and it's difficult not to) you take down its male half first.
Boss battles in Hitogata Happa are timed, and if the boss is not defeated when the time expires, it goes into "Error Mode", significantly increasing its attack speed. In addition, you only get one shot to destroy the boss while it's in Error Mode. Die even once (except for sacrificing your dolls as ((Action Bomb}}s), and it's Game Over.
Injury causes the main character to transform into an invincible, superpowered (and red) dragon during a boss fight in Breath of Fire: Dragon Quarter.
For reasons fans of the game aren't clear on, if you take a moderate amount of time to beat Xagor, the final boss of Final Fantasy Legend 3 (a SaGa game), several mouths will open on his demonic form, but he doesn't seem to get any stronger - in fact, he starts to be vulnerable to certain elemental attacks.
A few of the big bosses in City of Heroes invert this trope, although probably not intentionally. Diabolique would be the most memorable, as her new ability at low health is to pop up a force field bubble that prevents her from attacking or being attacked, giving players a nice breather to regain endurance and health at the cost of being a little irritating. Marauder and Bobcat have a more conventional glow power that reduces incoming damage at low health while still letting them attack. That'd be a conventional use of the trope, except that they lack long-range attacks, and when the glow wears off in three minutes, the power's crash means they'd end up with less health than before they used the power.
Romulus, on the other hand, comes back to life no less then three times. And every rebirth brings reinforcements and a nice healthy serving of stun. Not sure if he himself actually gets any more powerful, though.
Romulus eats one of his Nictus helpers every time he resurrects, taking their special power out of the fight and making things easier.
Ed the Undying in Kingdom of Loathing. He fights you no less than seven times in a row (the game won't let you enter his chamber unless you have at least seven adventures to spare.) He gets more and more beat up with each fight—each form has less health than the one before, to the point where you can easily beat the sixth and seventh in about one or two hits depending on level, and each form's portrait shows him in worse and worse condition. Eventually his unwillingness to give up despite how badly he's been beaten approaches levels reminiscent of the Black Knight scene from Monty Python and the Holy Grail. The pre-battle narration for his fifth form even includes the dialogue "C'mon, dude. This is ridiculous." "UNDYING!" "sigh."
However, though his Hit Points are halved each fight, his attack power does not change. And you can't heal between battles.
Fortunately, your familiar effect still has a normal chance of going off- including those that heal you after battles.
The Naughty Sorceress plays this both ways - she has three forms; the second is stronger than the first, but the third is a literal one hit kill if you have the right item in your inventory. And a one hit kill for you if you don't.
In The Legend of Zelda: Twilight Princess, Armogohma makes a comeback after you've seemingly defeated it. However, this "second form" just consists of a group of small spiders carrying its eye around, trying to escape you.
In Pikmin 2, the Waterwraith, a humanoid blob of water attached to steamroller-like wheels, does this when you finally get a fair battle against it. When its health hits zero the wheels explode into dust and the "Boss Defeated" music begins to play, but then cuts itself off and flows into an eerie-yet-comical version of the boss theme while the Waterwraith, alive but disarmed, begins panicking and running around the arena, trying to get away from you. It can't even hurt you at this point, and chasing it down and killing it is highlysatisfying.
All of the bosses in Ardy Lightfoot (except Beecroft) do this, though only as an indication that they're one hit point away from defeat. The most ridiculous example by far is the Final Boss, Visconti. After dealing with the apparent final blow while he is red, he suddenly turns blue and changes the strategy of the fight. Dealing enough damage to him in this form turns him back to red, though when you hit him like this this time, he turns grey and changes tactics again! It's not until you hit him when he's red for the third time when he's Killed Off for Real.
In the indie game Battleships Forever, ships go gradually weaker when armor plates, modules and gun turrets are chipped while not upgrading their remaining ones, sometimes resulting ships unable to attack the opponent after all.
The game it was based on, Warning Forever, didn't behave the same. Some otherwise empty sections (including the core) would fire bullets in a spread pattern if nothing is attached to them.
Bosses in One Piece: Unlimited Adventure do this, but Eneru takes this a step further: after his HP is depleted, he restarts his heart (something he does in the original series), making you fight him all over again this time with more bolts of lightning falling from the sky.
Rob Lucci does pretty much the same thing he did in series. After beating down his unassuming-looking human form, he uses the Cat-Cat Fruit (Model Leopard) and turns into a super-dangerous leopard-man.
In rRootage, once you reduce a boss's HP to a specified amount on its life meter, it will get stunned for a few seconds, then take on a second form with different, often more difficult attacks.
Abigail from the first Final Fight would occasionally turn red and charge at the player with a running punch, roaring as he powers it up. Subversion seeing as this charge is actually easier to stop than some of his others where he retains his normal skin colour and goes for a grab or a tackle instead..
Dracula in I Wanna Be the Guy parodies this. Normally when he shouts "Behold my true form and despair!", he becomes a hulking behemoth of a final boss. In this, he becomes A Waddle-Doo that can be taken out with one shot.
The Bosses in G-Darius turn brighter and brighter red as they take more damage, but that's just a way of letting you know how close they are to death. They tend to be a rather sickly, bright reddish-pink colour before they die
Other Darius games play this straight with later bosses and sometimes change patterns.
Although technically not in the same boss battle, Darth Vader in The Force Unleashed becomes excessively more aggressive and powerful once his armour has been severely damaged including the life support helmet the removal of which hastened his death in Return of the Jedi.
Every boss in Muramasa: The Demon Blade does this multiple times, partially because they can soak up a LOT of damage before being defeated. Each time you knock a boss' primary health bar down to zero, it gets angrier and its attack pattern changes.
A weird example is Ippondatara: You start with fighting his foot from the sky, then after a while you're carried up where you're confronted with his torso (which has more dangerous attacks). However during this phase you can turn him in the harmless Inosasao and deal a lot of damage to him.
In Tomba! 2, after you throw a evil pig into the pig bag at least two times, they lose their staff and do a different attack.
In World of Warcraft when fighting Arthas when he gets down to about 5% health he kills the entire raid in one move. Except that then everyone is resurrected and gets to wail on Arthas while he's held defenseless.
When Nihilanth from Half-Life is fully powered and healthy, it can either send a homing teleporter orb that summons some mooks into the battle and will teleport you into the Sealed Room in the Middle of Nowhere if it cathes you, or unleash a devastating bunch of ball-lightnings which can obliterate you and are hard to dodge, even though the player character can strafe at a speed approximately three times faster than a normal human's sprint. But, when you drain its energy and health, it opens its head, starts to moan and now can only fire a single pathetic ball-lightning at time, making it easier to get to its brain.
Two bosses in Tyrian implement this. The first boss plays this as mixed, where its armaments are destroyed but it still has an Emergency Weapon. The Savara V's boss has all its armaments destroyed . when low on health, making it impossible for him to attack you. (You can still die if you push yourself into him.)
Players who equip Dharok's Platelegs, Platebody, Axe, and Helmet in Runescape will have the ability to hit harder and hit more accurately the lower their health is. If players have only one HP left, then they can inflict ridiculously high amounts of damage, even being able to hit over 1300 points of damage in 1 hit, (For comparison, the absolute highest amount of HP any one player can have is 1100, and that's with the support of potions and augmentations).
Lucario becomes gradually stronger in Super Smash Bros. Brawl the more and more damaged he becomes. This is not an example of developers pitying your crappy playing, it's part of his strategy.
His power build up does cap at 200% so he won't become totally overpowered.
And if you happen to be dominating your opponents (having more lives than them, having significantly less damage than them), Lucario will actually get a little weaker.
Auron in Final Fantasy X gains this ability when equipped with his Infinity+1 Sword. The boost is substantial enough that he's able to hit the damage limit much much earlier than other characters.
In Mega Man Battle Network (the fourth through sixth games, at least), MegaMan does this when he is hit by an attack that does more than 300 damage at once or when he is completely paralyzed (as in, the directional pad is not repeatedly mashed in order to escape paralysis early). This angry/serious mode doubles the attack power of his next Battle Chip. Some of the later bosses in the fifth and sixth games have attacks that are so strong that they will cause him to enter this state repeatedly.
In Fallout 3, the Nerd Rage perk raises your Strength to 10 and your damage resistance 50% when your health falls below twenty percent.
Fallout 2 and Fallout Tactics also had the Adrenaline Rush perk which added 1 point of Strength when your health drops below half.
If that wasn't enough there is also nuclear anomaly, which makes you explode when your health is low, damaging enemies and allies alike.
The Ghost of She in Fallout: New Vegas creates three doppelgangers after you get its HP down to a third. Luckily, once the main boss is defeated, the doppelgangers die as well.
The Will Power ability in Soul Calibur IV boosts a character's stats when their HP is low. Its intention is to serve as a last gasp for the player, but with the right moveset it can help turn the battle around in their favor. Bonus points for actually turning them red.
In Tekken 6 when a character's health is reduced to around 10%, they enter the Rage state, in which their limbs glow red and they do much more damage.
In Samurai Shodown, getting hit repeatedly powers up your POW gauge. When it's full, your character becomes faster and stronger and, yes, Turns Red.
The Fei-Yen series in Virtual-ON games have a "Hyper Mode" that activates when down to a third of their energy. This makes them faster and more powerful. Some games will allow you to activate Hyper Mode at will, but doing so knocks your life meter down to a third.
A variation is Specineff, who has a Super Mode that makes him invisible to all attacks. There is a down side, however: If Specineff fails to kill the enemy in thirteen seconds, it dies instead. In fact, it's Leitmotif in Oratorio Tangram is titled "13 second warning".
Happens to Akiha's hair in Tsukihime. Her hair will turn from black to red when her demon blood is strong or if she's especially pissed off. This is generally a bad sign due to the high range, instant activation/transmission, high lethality of her power, willingness to kill if she has to and the only requirement being that she can see what she's attacking. She's actually not a very skilled fighter, however.
Paper Mario and Paper Mario: The Thousand-Year Door have badges that give Mario (and his allies) extra power or defense when their HP dips to 5 or below. Towards the start of the game, this is very useful since 5 HP is about half your max total.
Numerous Pokémon have abilities that activate either when they're low on health or suffering from Standard Status Effects, many of them being the starters, which may have Blaze, Torrent, or Overgrow.
There are also berries which are activated when low on health, such as the Salac, Petaya, and Lansat berries.
Two attacks, Reversal and Flail, increase in power as the user's health lowers, capping at an astounding 200 base power when health is below 4%. To put that in perspective, the top moves for almost every type (including Solarbeam, Fire Blast, Close Combat, etc) have a base power of 120, the few types who have an attack beyond that never going past 150, and the starter-only elemental beams Frenzy Plant, Blast Burn and Hydro Cannon cap at 150.
Also, almost all these attacks have side-effects, such as having to rest for a turn after using them, lowering the user's stats, having sub-par accuracy and so on. Reversal and Flail can be spammed with no side-effects whatsoever if the Pokémon who uses them goes first. And many of said Pokémon who can use Flail or Reversal also have access to Endure, which leaves the user with 1 HP if it would've defeated them... which is the perfect HP for Reversal or Flail.
In Pokémon Black and White, Darmanitan, with the secret ability Zen Mode, change form when their health drops below half. In Zen form they turn blue, change into the Fire/Psychic typing and increase their special attack and defensive stats at the expense of attack and speed.
Mario & Luigi: Bowser's Inside Story has Bowser get a status effect called 'Rage' when he gets hit too much (although chance can be increased by gear/badges), which makes him turn red and do about twice as much damage to enemies at the cost of lowering his defense.
In AdventureQuest, Berserker Armor gives you more strength the lower your HP is. Since Changing Clothes Is a Free Action, it can be advantageous to wear better armor for most of the battle and then switch once you're low enough that one or two attacks in Berserker Armor will finish off your enemy. Of course, if you time it wrong, you'll die... but that's usually no big deal.
While Gundam series isn't a game in the first place, once it's in Super Robot Wars or other similar games, Super Mode tends to be this, as it's required getting hit a lot or get to kill a lot of enemies to trigger. Gundam Exia follows its anime story and turns red in one of the cutscene in SD Gundam G Generation Wars.
Heck, many things in Super Robot Wars can be considered Turning Red— not the least of which is Eva-01's berserk mode, which activates when its HP is lowered to 0.
Enter SD Gundam Capsule Fighter: many of the playable mobile suits has their own ways of turning red, such as fortress mode that greatly increases one's defense.
A M.U.G.E.N version of Ichigo pulls this off when his health falls below 25% and his Hollow takes over. Also triggers a temporary fadeout and plays the appropriate clip from the anime. The screen continues to flash black, and the BGM changes as well. In this form, he becomes twice as fast and much more deadly, at the cost of his health dropping. He also gets a creepier voice and black smoke coming out of him, making him terrifying.
Jin Saotome in Marvel vs. Capcom would start to glow if he was the last member of his team standing and was down to a third of his health. This naturally made him a little tougher and his attacks hit a whole lot harder.
In The Incredible Hulk: Ultimate Destruction, the Hulk goes into an "Adrenaline Burst" when his health meter is depleted, during which he becomes stronger and has a few seconds to beat enemies and gain health orbs to save himself from death.
In Marvel vs. Capcom 3, if Phoenix is reduced to zero health (a rather trivial task since she has less the half the HP of the rest of the game's cast) with the maximum number of super meters stored, she heals to full, loses controlled of her Superpowered Evil Side, and becomes Dark Phoenix, complete with red costume and blazing aura.
X-Factor, which actually makes your entire team glow with a red aura. You whole team has temporary increased power, speed, and Regenerating Health.
This can also be a subversion, as you are able to use it as soon a you start a match. It still powers you up, but not as much as it would if you waited until you had one character left on your team of three.
League of Legends has several examples of characters who gain stats when their health lowers. The most notable is Tryndamere, who gains damage as his health gets low. This combines with a move called Undying Rage, which turns him red and makes him unable to die for 5 seconds, for a really nasty comeback.
Team Fortress 2 has the Equalizer and Escape plan, melee weapons for the Soldier class that buff damage for the former and speed for the latter as damage is taken.
In Street Fighter X Tekken, there exists a special desperation mechanic called "Pandora" where the user sacrifices their partner when health is low and end up getting a power boost as well as infinite Super meter (the story being that it's literally Pandora's box affecting warriors worldwide). It's signified by a power crazed animation at the start, followed by a transformation which induces jet black skin, glowing teeth and tongues, white markings and white hair and clothes and fur gain a purple glow followed by red eyes. The drawback is that it only lasts for eight seconds, and the timer doesn't ever stop and when it runs up, the user dies on the spot, regardless of remaining health.
The sub boss characters, Jin, Xiaoyu, Bison and Juri are in permanent Pandora mode but don't get any of the benefits.
Trope is played straight with Heihachi in where he literally turns beet red after chucking away his partner.
In Demon's Souls, Morion Blade and Clever Rat's Ring both increase your attack power (by 50% and 60%, respectively) when your HP is below 30% and is stackable For Massive Damage. It is further abusable by the fact that other weapon buffs or spell buffs also stack, making for truly fearsome damage output potential. The Dull Rat's Ring does the same, but increases your defense instead.
By extension of the above, Dark Souls also has this. The Blue and Red Tearstone Rings increase your defense and attack by 50% when your HP falls below 20%, respectively. While no weapons have the same effect, self-buffs and weapon buffs that increase attack power do stack, once again creating a truly fearsome damage potential, made obvious by the bright aura your character displays.
In Dungeons & Dragons 4e, all creatures have a "Bloodied" threshold, normally equal to half the creature's normal maximum HP. For many creatures this means little. For others, they will gain new attacks, a powerup, or special attribute modifiers when they hit this. However, others will become LESS powerful, losing auras or switching to substandard attacks. Also, some PCs have such abilities, and some powers from both sides can cause extra pain to creatures that are already bloodied.
The same applies in reverse, too: Monsters can do nasty things to Bloodied players, but Bloodied players can Turn Red in various ways themselves.
In the 4th edition, players who choose the right powers can become more powerful after taking enough damage to become "bloodied".
In Dwarf Fortress, dwarves in big trouble during a fight can enter a "martial trance", granting them that much more of a chance to survive. While rage increases attack strength, going into a trance does... other things. Other species can also become "enraged".
A number of the evilities in the Disgaea series activate either when a character is in critical health or when allies are defeated, granting benefits like increased attack power or evasion, or immunity to certain attack types. The most powerful of them is One-Man Army, which doubles a character's stats if they're the last one alive on their side, which can make them extremely difficult, if not impossible to kill depending how high their stats were to begin with.
This can be (intentionally) exploited by equipping a character with a "Dumbbell", which sets the character's max HP to 25% of normal, keeping them in Critical HP — and, therefore, Turned Red — for the entire fight. One-Man army requires that the other 9 guys on your team be dead first, so you can just bring along 9 cheap, disposable units to use for that purpose (such as lv1 prinnies, who can all be taken out in a single turn and resurrected later for pocket change)...If a character has a "Critical HP" Evility with a Dumbbell, AND One-Man Army, they can be quite the Game Breaker. One can also use gear/Evilities that increase stats based on how many allies have died for a similar effect.
Super Punch-Out!!: Narcis Prince is a vain British boxer who fights very defensively. Punching him in the face angers him, making him fight more aggressively (and his punches cause more damage). But he fights carelessly, leaving himself open for more punishment.
In One Piece, chief prison warden Magellan has a power to produce poisons. While an exceptionally dangerous individual, he usually limits himself to avoid needless deaths among prisoners or jeopardising prison staff. If the enemy is too quick to engulf and one-shot, he will resort to Gradual Grinder tactics. However, once the heroes instigate a major breakout, he goes all out, animating a wave of toxic slime into an Advancing Wall of Doom so poisonous it saturates anything it touches, spreading through nearby walls, objects and even people. Bonus points for actually turning red.