Demonic Spiders are known for providing more action than the player bargained for.
Hawks are ubiquitous in the later levels of the NES Ninja Gaiden games. They take away three squares of Ryu's life bar, more than bazookas and some bosses. They are small enough to require incredibly precise sword work and often mob the player. Ninja Gaiden also has a wonderful quirk: If you get hit, you have to sit idly by with a thumb placed somewhere convenient until Ryu lands. This means that, if you get hit by a hawk when attempting one of the games' innumerable deadly jumps, you're dead meat. And with other enemies to distract you, they are the cause of more than a few thrown controllers.
Even worse, the NES games have no Mercy Invincibility, and enemies can respawn if the screen scrolls far enough forwards or backwards. Imagine getting hit by the aforementioned hawk, and being knocked back, causing another hawk to spawn and attack, causing the original hawk to spawn and attack...Juggled to death doesn't even being to describe the infuriating circumstances.
The dreaded Stage 6-2 from the original has the Demonic Flying Ninjas who throw hard-to-see shurikens at you, in collaboration with the Goddamned Hawks. That One Level has been the cause of many broken controllers and TVs.
The Boogie Men in ToeJam & Earl. Invisible, faster than you, and they take off a good deal of health. Even worse in the sequel, where you have to catch them all, and they often come in packs of 3-4. Still invisible, meaning you'll think you've caught them all and then you'll walk forwards and suddenly hear "BOOGIE BOOGIE BOOGIE."
The ice cream truck is fairly rare and technically outmaneuverable, but is normally much faster than you and can take half your health or more (depending on your max health) in one squish.
Or the Lawnmower Guys. They're fast, they're smart, and thanks to the lack of Mercy Invincibility, one hit and you're dead, no questions asked. As if that wasn't bad enough, they'll still follow you after you die, so once your after-death invincibility wears off, 9 out of 10 times you'll be dead again. Cue several Genesis controllers being thrown against the wall in pure frustration.
In Kid Icarus, at the end of each world is a fortress level. To get to the ultimate boss always requires passing through at least one room featuring the dreaded Eggplant Wizards, creatures who liberally toss eggplants around the screen. Being hit with an eggplant turns Pit into an eggplant with legs, unable to fire weapons. Of course you can't defeat the boss in this state. The only way to remove the curse is to travel about a dozen rooms out of your way to a "Hospital" room, dodging enemies all the way since you can't shoot them...and then go back to the boss room, hoping you can avoid the eggplants this time.
While Eggplant Wizards are the worst of the Demonic Spiders in Kid Icarus, they are by no means the only ones.
In the horizontal scrolling levels are Plutons. They are Invincible Minor Minions, and if they touch you, you can say goodbye to one of your powerups. And since they come by the dozen, you need to be very good at dodging them or you'll lose them all. On the bright side, if they're stolen, you can buy them back at the black market if you're willing to pay up to 700 hearts.
Dark Castle: The important thing to remember about this game is that you get paralyzed a lot. Run at a wall too quickly? You stand there teetering with stars in your eyes for ages. Walk off a half-foot ledge instead of hopping? You trip and end up flat on your face. Jump off a head-high platform? You're overcome with dizziness. While you're standing there wobbling around like an idiot, rats, snakes, imps and bats are happily chomping away at you every time they pass, and any larger enemies that are looking the other way or knocked out (you can't kill larger enemies) can kill you in a single shot. Plus, whenever you're in this state, you always have to stand up almost immediately and stay that way, regardless of the traps you are placing yourself in the path of. Honestly, Prince Duncan is his own demonic spider.
Not only does Odin Sphere have both the Goddamned Bats in the ice and fire stages and a heaping helping of Those Twelve (Give Or Take) Bosses, but there is one enemy in the Titania stage that is the bane of all existence: the red wizard. The standard wizards aren't that much of a problem; sure, they teleport around and can sometimes inflict status ailments, but just knock one of the floating swords from their sword spell back and you can finish them off quickly. Red wizards, however, are more resilient than that. They'll try to beat you down with every status effect under the sun, and because they teleport so much it can be hard to catch up with them before they use their sword spell or teleport again. Not to mention they can summon slimes, which only take one hit point of damage from any attack that isn't magic-based and are capable of locking you in place and slowly draining your hit points until you are dead—and by the way, the first time you meet them, unless you've been warned in advance and stocked up on Napalms, you have no way to hurt them. Oh, and red wizards often show up in groups of three or more. Between this and the mini-bosses, Titania is pretty much That One Level.
The fire stage also has the lava monsters. Kill them and they explode. If you're caught in the explosion, you die instantly. And the boss of the same stage can also kill you instantly, and when he isn't doing that he summons lava monsters. Good luck not dying instantly.
Not to mention the Sorceress Fairies who show up in Ringford (any anywhere else you fight against the Vanir). Not only can they spam fireballs at you, they can heal anything nearby to full health in seconds, including themselves. Deadly when they're floating above hordes of ground enemies who make it difficult to take them out, but particularly bad during the boss battle between Mercedes and Oswald, where they can heal the boss while he's powered up and you're forced to continually run away without being able to retaliate.
Sludges, which come in large groups, are fast, do tons of damage with their attacks, have exceptional reach, and have a counterattack (basically, they turn into an unhittable puddle of goo and do a Shoryuken-esque uppercut) which is nearly impossible to avoid that they use nearly every time you hit them. Your best chance against them is to frantically try to nab one yourself and spam that counterattack yourself.
Battlers, who constantly block, also come in large groups, and have a very painful special attack which they love to spam.
Crash: Mind Over Mutant has TKs, which have ranged attacks that are impossible to avoid, a annoying melee attack which they are invincible while using and which has a large area of effect, and can throw you off your titan, leaving you vulnerable while you try to re-jack your titan.
The Demon Dolls that show up in the third level. They're quick, hard to hit, have a projectile weapon, spawn like crazy, take several hits to kill, and have a 50% chance of lighting on fire and chasing you around after being killed.
Blobs that shoot jelly globs into the air that, if they hit you, drain three bars of health while leaving you unable to use weapons or items. And they are invincible to anything except popsicles and fire extinguishers! Not even the Flamethrower/Monster Potion/Pandora's Box can touch them!
The Chainsaw Maniacs, durable enemies that can chop through most obstacles and actually track you across the screen. It's possible to kill one, but it takes obscene amounts of damage to do so and most of your weapons can only stun them. Unless you have a monster potion, it's best to not bother, because you'll be fighting several. And to top it off, their ability to cut through obstacles means they can break through to victims other monsters can't even touch.
Fishmen that like to leap out of the water and land right next to victims, then kill them. They're also infinite respawners.
But worst of all are Werewolves. Not only do they pursue you across the level (which includes jumping through walls), but most Werewolves are actually transformed tourists, who you're trying to rescue. In levels that turn to night, you must rescue all Tourists before it becomes night-time or else they'll become Werewolves and you lose one of your victims. The time limit for this transformation tends to be frustratingly short as well. At least you can one-shot them with the silverware. (In fact, 90% of all enemies in the game can be destroyed by one or two shots of the correct weapon).
Vampires up randomly, take obscene amounts of damage, and can pretty much kill you at will, if it chooses to fight you rather than stand and soak up your ammo. Not to mention their annoying habit of flying away before you can finish them, resulting in a massive loss in weaponry with no reward. On top of that, their bat attack is pretty much unavoidable. It says something about the game's difficulty when the Demonic Spiders can throw Goddamned Bats at you.
Giant ants climb over walls and can take a large amount of punishment from most weapons— only the rare bazooka and rarer Martian bubble gun can one-shot them. They can pick up unclaimed ammunition and carry it back to their ant holes, never to be seen again. And like most enemies, they respawn endlessly.
The heavily armored, quick, and downright nasty Land Lobsters from the first game qualify, unless you throw the wrench.
The Y.E.T.I.s from Going Commando's Planet Grelbin; they spawn from out of nowhere (and in large numbers, to boot), have no concept of Mook Chivalry (often ambushing you in groups of ten or more), and are card-carrying members of the Lightning Bruiser club. You could try running from them, but they love spawning right in your path and often appear around Arctic Leviathans, which must be killed for Moonstones. That leaves killing them, but there are a few problems with that as well: first, very few of your weapons are effective against them, the ones that are either have low ammo clips or have ammo that must be bought at a vendor, and for the weapons that don't, you're thwarted again by the fact that there are very few ammo crates lying around the massive ice field you must traverse. As a result, you'll find yourself making frequent trips back to the vendor, which often means running back into the Y.E.T.I. mobs you just fled from. The Y.E.T.I. was considered such an Epic Fail from a design standpoint that Tony Garcia, who programmed the things, received a hand-made sculpture of a Y.E.T.I. "congratulating" him for creating the worst aspect of the game from his fellow workers. This eventually evolved into the "Snowbeast Award" that was given to whoever designed the worst part of a released game (incidentally, and perhaps unsurprisingly, the missile turrets described below were the winners of the second Snowbeast Award).
Qwark's Hideout in Up Your Arsenal features heavily armed catchphrase-spewing Qwark Bots. They deal ludicrous amounts of damage with a very high rate of fire, have more armor than any other enemy of their size, and can even survive after being reduced to nothing but a skeletal torso with one arm, often lurking in this state until you pass the cover they're hiding behind to swipe at you. They also spawn in dense packs.
Up Your Arsenal also features Tyhrranoid Missile Stations, which often appear in Galactic Ranger missions. They're not too bad when you're piloting a Hovership, as the missiles can be easily dodged. When you're on foot, however, they'll shoot missiles that fly directly into the ground and explode in a shockwave that hurts you unless you jump over it, and they will continue firing even when you're in the middle of fighting other enemies.
The beam turrets in Deadlocked. Tiny little laser turrets that drop in at random points, and are usually overlooked right until you get a laser beam in your back.
The giant flying robot roaches, aka Laser Backs (also from Deadlocked), are a) infuriatingly hard to kill, b) have a psychotically high rate of fire, and c) are one of the few units that make life difficult for you even when you've turned them to your side with the right omega mod. (Usually, brainwashed enemies are kind enough to try and aim at the enemy; the Laser Backs simply spam ammo everywhere, and God help you if you get in the way.) Pretty much the only way to keep them from reducing you to powder was to wait until they briefly stopped shooting, then spam them with a high-knockback weapon. Oh, and your helper-bots have no idea that these things are unspeakably annoying and will instantly get mowed down by them.
Their sorta-predecessors, the Guard bots from the third game, are this too. They have the same attack method—-that is, two guns that More Dakka you into the pavement—-and take buckets of damage.
Red soldiers. If one pops up, you better shoot it ASAP or prepare to hide, because their first shot will hit you if you don't hide. Fortunately, in Time Crisis II onwards, their killing shots have a distinct circular mark on them (known as a "Crisis Sighting"), are colored red, and, in Crisis Zone, are accompanied by a warning beep.
Even worse are ninjas, which jump around all over the place, stay behind cover half the time, sometimes throw knives, and tend to pop up at close range and give you a facelift before you have time to react, especially in Crisis Zone. And many times there's no warning before they attack.
Enemy ace squadrons are one thing, since at least in a fresh run (as opposed to a SP New Game) you won't have a comparable plane until the late-game... it's another when they're outmaneuvering you, consistently getting on your six o'clock and seriously threatening you in the late-game with a starting plane. (See Alberto Lopez, aka "Espada 1" in Ace Combat Zero.) A "late-game" version would be the fact that planes like the YF-23, the F-22A, the F-117, and the F-35C have fade-from-radar stealth that greatly increases their deadliness when flown by the enemy... but has no apparent effect when you pilot them.
Any planes that require more than the standard two missiles to shoot down. The most notable instance would be the C-50 cargo planes from Ace Combat 5's arcade mode, which require up to ten to shoot down and, unlike other similarly unarmed planes, don't give you any back for killing it.
The hyper-competent, super-agile drones you have to dogfight in the fifth level, "OFF Certification", while in OFF Mode. Thankfully none of the fuckers can kill you since it's essentially a tutorial unless you drive into the ground like an idiot, but three drones that look like Global Hawks with satellite dishes can outdance a hyper-maneuverable experimental jet that never saw combat in real life and arguably give the most difficult dogfight in a game purportedly meant to be about air combat.
The Su-37's from the 14th mission - highly agile and maneuverable planes that can avoid your missiles effortlessly and continue to launch at you, and then the mission's objective forces you to ignore them entirely and go fly a complicated pattern through a wall of anti-aircraft fire, all while the Su-37's are still chasing and firing at you.
Donkey Kong Country 2: Diddy's Kong Quest has Klobbers, Kremlings that pop out of barrels and try bouncing into you. They're not so bad, but whenever the black ones bounce into you, a life balloon appears. They come from your total. They can end your game if they bounce into you enough. There's also the red ones with TNT barrels, which are a One-Hit Kill. The worst thing ever is the secret That One Level Klobber Karnage, filled with both of these and annoying, long sections with spinning barrels over Spikes of Doom.
Donkey Kong Country 3: Dixie Kong's Double Trouble! has Lurchins, sea urchins that absolutely blanket the underwater levels. In most cases, being underwater means you can't attack them at all, leaving you the sole option of getting out of the way (and in one level, they sweep upward from offscreen so fast that you have to know in advance where they are, or you will get hit). The only way to hit them is to ride/become Enguarde the Swordfish...and hit them at precisely the right moment; if you stab at them when their shell is closed, you get hurt, and their opening-and-closing speed varies from Lurchin to Lurchin.
Perhaps worst of all is the Metal Leo, which while it only appears in the last level, is a scaled-down version of the previous boss.
Special mention should go to Cromartys, the black-suited versions of normal mooks. They both have massive damage reduction and can roll away from your combos unless stunned, require dodging of a relatively difficult combo attack to stun (during which other enemies will also be attacking) and often appear in groups, armed with bazookas and non-deflectable homing missiles.
It got worse in the sequel, where they appear as ninjas with even worse homing fireworks and a new firework bazooka which, unlike the old one, doesn't let you reflect the shell back for a free stun. So much for the Conservation of Ninjutsu.
Escort Missions make normally not so bad enemies like Jalthis this trope in Wing Commander but Hhriss from the Expansion Pack are real Demonic Spiders with the agility of the Salthi, the firepower of the Raptor, and the armor of the Jalthi, only with a more balanced front/rear distribution. They are ridiculously powerful.
In Star Wars: TIE Fighter, the A-Wings. Here's a quote from Game FAQS: "The A-wing was a nasty little foe and the hardest rebel starfighter you could come up against in the original TIE Fighter (sans expansion packs). For those not well-versed on their Star Wars lore, the A-wing is a Rebel interceptor, a fast, dagger-like ship armed with laser cannons and concussion missiles and with engines unmatched by anything in the Empire's basic arsenal. In TIE Fighter, these little monsters were blindingly fast and incredibly maneuverable, capable of outrunning and outflying every player ship until the TIE Advanced found its way into the storyline. This incredible speed, combined with its compact chassis, made the A-wing incredibly hard to destroy. Even the anti-starfighter concussion missiles, usually a guaranteed kill, were useless against the A-wings, which would nimbly evade the missiles until they self-destructed. Not only were the A-wings difficult to kill, they were also dangerous, as the concussion missiles THEY packed could shred your unshielded TIE faster than you can say, "I'm your father"!"
Near the end of Psi-Ops: The Mindgate Conspiracy, Aura Beasts begin appearing. These start out invisible and invincible, only becoming vulnerable when your character alerts them. They then appear at point-blank range and proceed to fly into your character's face at high speed while attracting a small swarm of others. While they are quite easy to kill, they have a grab attack which takes a massive chunk of health off your character.
In Ninja-kun: Ashura no Shou, most enemies are like the ones usually found in the rest of the games. A short time you'll face larger "boss-like" enemies, including boss skeletons, which unlike the others can kill you if you aren't careful. However that's just the beginning, as soon after you'll find bats that instead of being annoyances are one-hit kill, actual spiders that show up spontaneously in the middle of a room and are ungodly fast and many other things that will kill you.
Pretty much any enemy in The Legend of Kage is a demonic spider, due to the game's abundance of Fake Difficulty. Kage is a One-Hit-Point Wonder, has a near-zero range sword and hard-to-aim ninja stars, and enemies randomly drop from the sky, making it nearly impossible to avoid running into an offscreen enemy or their projectile attacks. Thus, the whole game is mainly a Luck-Based Mission.
The cougars. They're insanely fast, agile (making it hard to shoot them) and aggressive, seem to hunt in packs, and can kill you and your horse with ease. There's a reason some people refer to the cougars in this game as modern-day velociraptors. This is particularly notable due to the fact that cops and outlaws can put an entire magazine of Colt semi-auto bullets into you and you're still fighting fit, but one swipe from a cougar puts you on your ass, and one more has you seeing (red) dead.
The Mexican bobcats are cowards that more often than not run from Marston when you get close. But if they do decide to attack you...
In RoboCop 2 the arcade game, in the chemical plant stage, there are the mutants who run straight at you, electrocute you as soon as they touch you (so you can't "bodyslam" them like you can with other toughies), and take quite a few shots to kill, and appear with the same frequency of all mooks.
Alien Swarm has the Parasites, which are very small, very fast, and spawn in massive numbers. This is already a formula for a nasty enemy, but what really sets them apart is their method of attack: They attach to and drain your health very quickly, and the only way to get them off is for The Medic to spend some of their limited healing items on you. Did I mention there are loads of them? And they're so small and fast that hitting them is mostly down to luck? And there are like a million billion zillion of them? Well, it bears repeating. To make matters worse, being killed by a parasite release several more parasites upon your death, making the situation go downhill VERY fast for the rest of the team. Once you've unlocked the flamethrower and the telsa gun, the parasites get a bit easier to deal with, but not by a whole lot.
Streets of Rage 3 and the mohawked martial artists (Tiger/Leopard/Fabio). They move fast and hit hard with rapid series of kicks to the face and flying leaps. They sometimes arrive on the scene falling directly from above, feet poised to hit your face on the way down. They can (and often will) block almost any conventional attack you throw at them, including special attacks, with a mocking lack of visible effort. Their only weakness is their susceptibility to getting thrown or body slammed... if they are alone. When in groups, they exploit the stacking sprites as a team and form an unapproachable phalanx of forearm block and Rapid Fire Footwork that is immune to most tactics and retaliates without pity from beyond melee range using leap-kicks.
The Haunted Mansion has the gargoyles, which are really big enemies that could only be harmed with a fully Charged Attack. Which is hard, because they are also Enemy Summoners, and said enemies are much faster than you, and when they hit you your charge stops . Luckily, the Beacon of Souls gets more powerful, lowering the demonicness.
The sequel keeps the Archers (and their faster palette swaps the Freischutzes) while introducing Liliths and King Specters. The former will teleport away from you if you so much as start to move in their direction, and if not stunned as soon as they rematerialize (which requires you to anticipate where they're going), release a swarm of Invincible Minor MinionGoddamned Bats from under their skirts. Also, they're invincible while doing this, and never appear by themselves. The latter surround themselves with their kids (which possess nearly as much health as the parents) before dive-bombing you repeatedly-and the approach to the last boss has you facing two of them at the same time.
Any enemy who can poison you in Tomb Raider III. If poisoned, you'll keep losing health until you use a medi-pack. The game has a good amount of enemies that can poison you, thus you may keep wasting your healing items just to cure poison. The future games still have poison effects, but done much less.
The literal spiders, particularly in Tomb Raider II, are small and hard to hit, and typically come in swarms in dark areas, so you will often get nibbled within inches of death.
Similarly to the poison example, any enemy that can set you on fire. Unlike poison, burning is a death sentence unless there's water in the immediate vicinity. Again, II has by far the highest amount of these, and they like to quietly sneak up on you with deceptively long range flamethrowers.
Psychonauts features psychic, mutated rats on the upper floor of the Thorney Towers Home For The Disturbed level, which scamper directly at Raz and explode, not only causing impact damage but also leaving a cloud of harmful Confusion Gas. They come in swarms which home in on Raz wherever he happens to be, and by the time you hear the distinctive squeaking and scampering noise they're usually right on top of you. Shooting at them causes a chain of dangerous explosions and fills the screen with Confusion Gas. Since the gas messes up Raz's powers, it's almost impossible to stay shielded during a rat attack. On top of this the sound is glitchy, so half the time the warning sounds can be heard when there are no rats present—Paranoia Fuel if there ever was one.
In Ys Book I and II, many of the monsters in the lower levels of the mine and upper levels of Darm Tower fit this trope. Especially bad in the mine with limited visibility. Also bad is the second mirror warp maze in Darm Tower, which is full of Elite Mooks which can kill you in two hits even if you have the Battle Armor.
In Book II, the Mine doesn't have limited visibility, but there's no way of knowing if there's a crowd of monsters on the other side of a doorway, which can lead to unavoidable deaths. Better Save Scum.
Hobbledy in Folklore, especially when it appears in groups. It uses a continuous stream of electricity to damage you. This can also paralyse you, leaving you frozen in place and helpless as the group take turns blasting you, effectively keeping you paralysed and making it so all you can do is watch while your HP drops like a stone. Even if you manage to get one of their ids out, the others WILL NOT let you absorb it, standing in your way and blasting you if you even so much as THINK of pressing R1.
The Ascidia and Agar-Agar have the distinction of being some of the odd folk that can both regenerate their own health and multiply if given enough time (thankfully capping at 4). While some players like to bring out their toughest Folk to deal with most situations, these things require constant attention with from smaller attacks lest they start regenerating again. This is far worse for Keats, who requires some SIXAXIS tricks to absorb their Ids, creating a pretty high chance of being interrupted (Ellen can just absorb them normally). They also appear to take massive damage from Keats' Fire folk, when in reality this will rarely be enough to kill them.
The Bougainvaillea that Keats fights in the Netherworld Core are some of the most unforgiving Folk the game has to offer. Just one of them throws out a high-damaging, quick-moving, bouncing ball of fire that practically homes in on the player with sadistic accuracy, and they tend to appear in packs. More dangerously, their bodies will often project 3 of these projectiles when they've take enough damage, cheap-shotting you during your own offensive. No matter your skill at dodging, it's rare to walk away from an encounter with these creatures without losing at least some health.
The Gladiolus, also found in the Netherworld Core can be exceptionally terrifying for the simple fact that when they come after you, nothingyou throw out can stop them, and their damage-output is monstrous.
Amagon has these in spades. Fast-flying birds that suddenly appear while you're jumping, erratically-moving wasps that shoot spreadfire, jumping mushrooms, Spiny-type things that drop from trees, teleporting robots, etc. In all cases, you die in one hit unless you're Hulked Out.
Any enemy with a level 3 Full Screen Tech, such as Wolf Devils, Dark Lords, or Nightblades. Which is a shame, because all of the final dungeons are crawling with them. There's a very real chance that a single Full Screen Tech coming from them can wipe your entire party in one shot, unless you've been doing some serious level grinding (and even then, it'd come close). The biggest problem about these multi-tech bastards is the fact that they seem to use those techs completely at random. If you have very bad luck, all 2 or 3 of them will use their multi-techs at once, giving you no time to heal in-between. This usually results in an instant kill, even with strong characters and "Defense Up".
Wolf Devils would count even without Sky Dance: they're fast, strong and attack in numbers. Even with nothing but physical attacks, they can kill your charactersó-especially your CPU-controlled ones-óbefore you even notice how low your health is.
Spawn: In The Demon's Hand has the shotgun-wielding skeletons in Boss Attack Mode. They're capable of taking off 80% of your health with a close range blast and appear constantly, sometimes in groups of three or four, while you're also attempting to keep an eye on the (usually dangerous) boss that you're expected to kill. Just to rub it in, they're small grey humanoids in largely grey levels, and as such are often hard to distinguish from the background. They are at least slow, but characters with low mobility can still have trouble escaping them.
The Godfather game has Tommygun and shotgun users who can shred you very fast, especially given that they first appear quite early on when your health is still quite pathetic. They can tear up cars rather quickly too, so tough luck if you encounter them without a nice sturdy wall to hide behind. Plus since you are only one man, they are likely to flank you. At least you can pick up their guns after you pop them with a Boom, Headshot!... but still, prepare to die often.
Goblin Bat Riders and Giant Bats. Both are immune to all spells except fire based ones (and there are only 3 fire based spells), they're fast, they do a lot of damage, they infinitely spawn bats (including upon death). Luckily, they are rather rare.
Mages, which attack in swarms, hurl fireballs, Teleport Spam, summon golems (fire and ice) and are completely immune to any form of magical assault. Luckily they are fairly easily to kill with normal weapons... if the golems and fireballs don't kill you first or if you can catch them.
In the Brutal Bonus Level, you get the Buttes and the Mesas. The Buttes are tiny Pit-looking things that either fly around randomly, shoot arrows, or wait on the ground and try to charge you. They do tons of damage, can quickly delevel your weapons, and the ones on the ground can block your attacks. Even the Spur. The Mesas are giant Buttes that throw blocks at you for a ton of damage and have far too many hitpoints.
In earlier levels, there's the Sandcrocs, which deal huge damage, only pop out when you're standing right in the path of their waiting jaws, and slide back under the sand before you can get a chance to kill them. Until you lean how to tell where they are, expect a lot of Sandcroc-related deaths.
The Elites. They're ridiculously fast and have shots which linger around for a while.
From the Bokor levels is the equivalent of the Elite, the Cuttles. They have the trademark ridiculous speed and powerful attacks of the Elite enemies, except they use melee attacks. Also, due to the different "wave" style of the Survival mode, they gain tons of health very fast.
Flying MTs in Armored Core games pre-4 and For Answer aren't as predictable as fighter jets nor as slow as helicopters, both of which you can kill with a missile or two, charge at you guns blazing at a low altitude, fly in erratic patterns, frequently come in packs of up to three and drop other enemies for you to deal with. And they all have enough health to bar the player from just shooting a few missiles and scooting. If you aim up to engage the little bastards, you get blasted at by their ground support. Neglect aerial threats too long and you get shot at by them, plus the ground targets. They tend to appear in droves and one mission in Silent Line in particular quickly turns into That One Level because you must face a boss right after fighting them off, PLUS the Kill Sat that's firing at you while they're chipping away your health steadily.
Fester's Quest: In the alien ship near the end of the game, you encounter the Beholders, cyclopean floating heads that can follow you through walls, take alot of damage, shoot volleys of fast projectiles, and fire back when hit. Better stock up on smart bombs and other items before entering the ship, as it's a Point of No Return.
Fortune Summoners is basically made of Demonic Spiders. Each dungeon has at least one variety among its varied mook fauna (usually one of the mage-type enemies. If the mages are not that bad, it'll be the beefier fliers), sometimes two, and every time you enter a new dungeon, you have to be very careful, because there is always some kind of new enemy that can easily get you stunlocked to death in seconds. Special mention, however, has to go to Ghost Wizards. These guys usually come in pairs, can heal themselves and other monsters (including each other), can dodge by becoming immaterial, tend to fly just outside of your sword's reach, and can get through the walls... and pack a spell that does very respectable damage, is virtually undodgeable, and can be cast from inside the wall. No, you can't really attack them while they're inside the wall. Yes, it is precisely as annoying as it sounds.
The Flame Parasites from Evolva. Not only do they have the ability throw huge fireballs towards you for massive damage, but they always catch fire after killed and explode after a few seconds. Depending on the battle, you may not be able to get far from the explosion, since you may still be fighting the remaining enemies.
The Hobbit has literal spiders starting with the Mirkwood level. They come in two variations: the big tarantulas, which are actually less awful because they can't poison you, and the tiny green and slightly larger red ones, which can poison you. There is no way to cure poison except for an antidote, and they come in swarms. You will often exhaust your supply of antidotes after one fight with these things.
Thief: Haunts. Undead enemies are already annoying since they can't be knocked out, and the weapons capable of killing them are extremely limited. But zombies are slow and ghosts are easily avoidable, not to mention rare. Haunts on the other hand are semi-common late-game foes, have incredible hearing, move much faster than anything else, and are even more skilled in swordplay than the human guards are. It's possible to beat them in a swordfight... theoretically. Almost all players prefer to simply run like hell if one spots them.
The crawling titans from the Attack On Titan Tribute Game are even worse than their regular abnormal counterparts. Unlike regular titans, these things keep low to the ground (hence the name), making it tough for many players to target the neck and kill them. They are VERY fast on the ground too, making for an ugly time for any player caught on ground level with one and further making it harder to target the neck. Take to the air or get on a building? These things will quite happily leap up into the air to nom you. The only character who can take them out easily is Eren in Titan mode.
In the video game adaptation of The Crow: City of Angels, every enemy with a gun is one of these. Every. Single. One. Due to The Problem with Licensed Games, you are at a severe disadvantage because of the game's many faults. If you can manage to pick up a gun and shoot the enemies (a challenge in and of itself due to the game's clunky, delayed controls, terrible camera angles, and poorly done 3D physics), you only get a few shots before you run out of ammo. The enemies, on the other hand, never run out of ammo and it's guaranteed that they'll hit you far more than you'll hit them because they don't have to deal with the aforementioned faults of the game. They can, and will, completely drain your health in literally two seconds.
The serpents in Overlord I are just obnoxious. They take a lot of hits to kill, can kill multiple minions in one attack, usually hang out in the water where only Blues - your least impressive combatants - can go, and occasionally dive into the water and reemerge in a slightly different location, accompanied by the deaths of multiple minions. Thankfully, you fight very few of them.
The tri-missile launchers in the mecha city and headquarters stages. While their attack is weak, they have a very difficult to hit hitbox (that is higher than you would expect, appearing just above the launcher) and they stagger Godzilla, making hitting them a chore. They also block Godzilla's progression, and battling them leaves you vulnerable to everything else on the screen as well.
These things have nothing on the dreaded inferno craft however. Practically unkillable and slowed down by nothing, it sloppily homes in as it moves across the screen. It ignores mercy invincibility and inflicts constant damage as it slowly passes through its target, and is easily capable of killing even high level players in one shot if it lands a hit. To top it off, even when the screen isn't already swarming with enemies (which is a rare occurence), the inferno craft is tricky to dodge. Since it appears as early as Mars, it's also typically the first thing to kill Godzilla in the game.
Haunting Starring Polterguy: Some of the monsters in the underworld are very strong and hard to dodge. The arms/fists from the walls hit you very hard, and those from the pits suck you in, and your energy is very limited (you can only lose the game here).
Bangai O Spirits has Longai-o, enemies who, like yourself, can unleash massive barrages of missiles capable of wiping their enemies out in an instant, and they will do it at every opportunity. Usually in response to your own missile barrages. And they like to attack in groups, launching terrifying walls of death at you faster than you can counter them. Have fun!
The Heavily Armored Mook enemies in the Uncharted series. These are mercenaries and soldiers clad head to toe in body armor who slowly advance toward you armed with either an assault rifle, or much more commonly, a shotgun, which will kill him instantly if they get too close. You cannot fight these guys hand to hand either, as they'll instantly knock you away. In order to take care of them, you must either unload multiple rounds of gunfire to kill them, or use multiple well-placed headshots to first knock their helmet off and then finish them. Even with those ways to kill them in mind, they can be very good at sidestepping away from your line of fire, and when they get to close you'll be forced to flee further away in the hopes of finding more cover (which just leaves you open to more gunfire)... unless of course you're backed into the very end of a corner and there's no where else to run, in which case you're effectively screwed.