In the Taito arcade game Rastan, at various points a flock of bats would come out, through walls and platforms. They would seem to get faster and faster, and not just come in a flock—they would come at you from all angles: above, below, fore and aft. Your barbarian with huge muscles and two-handed sword would soon be nickel and dimed to death by, yes, Goddamned Bats.
Drawn to Life have Shadow Goo Monsters. Particularly annoying in the second boss fight, where they keep spawning out of thin air, interrupting your jumps and whittling down your health meter as you try to attack the boss.
Lego Star Wars went from a light, enjoyable platformer to an exercise in precision controller-hurling over the course of the Dagobah level in The Empire Strikes Back, thanks to the incessant bats circling the swamp. Between Lego Luke's spastic lightsaber "technique" and the bats' instinct for homing in on him, the simple act of crossing a swamp became something slightly more difficult than, say, defeating a Sith Lord in one-on-one combat.
Dementors from LEGO Harry Potter: Years 1-4. While they usually only appear in Year 3, they're infurating while they appear. Their main attack drains health and has a long attack animation, and they appear in swarms. It's not uncommon to be attacked by three at a time, and after getting attacked by one, another one moves in until you die. They're only killed by one spell- and said spell has a long wait between attacks and is lengthy unless you jump and use it.
The infamous Red Arremers of Ghosts N Goblins fame. If you try to hit them straight they just dodge your weapons and fly away where you can't even think of getting them, have an annoyingly precise projectile attack, and fall on top of you with VERY QUICK dashes, helped sometimes by the game's glitchy walls and platforms. They're the number one cause of putting people out of the game, and in later levels there's plenty of these red bastards; since attacking them randomly is a no-no, only a patient and precise player can overcome them. And guess who was Capcom's Final BossMascot Fighter in SNK vs. Capcom: SVC Chaos, and made a comeback in Ultimate Marvel vs. Capcom 3?
The frogs, aka Kellos. Once you hit them once or get in range, they start jumping towards you and become very difficult to hit again before they kill you.
The cobras in Adventure Island, unlike Wonderboy, shoot erratically-moving venom projectiles.
Whalers (swordfish) tend to spawn while you're navigating tricky platform jumps, and come at you almost non-stop in the later ocean levels.
In the later levels you had to fight through hordes of literal demonic spiders (eg 7-4), if you die and lose your axe beforehand and there is no replacement readily available after the Point of No Return, Continuing Is Painful, and in fact the level may be Unwinnable without the axe.
Those ants in Bugdom. What's so annoying about them? It's the way they come back as invincible ghosts when you kill them in later levels. Fake Difficulty meets Demonic Spiders, and they get along well enough to gouge huge chunks out of your sanity. About the only reliable way to deal with them is to get them to throw their spear, dodge, and then run like a scared little girl while they're retrieving it.
The freakin' Hammer Brothers in the original Super Mario Bros. are, by far, the most dangerous mooks in the game, including the fake Bowsers, which throw hammers on World 6 and on, but at least their jumps are bigger and floatier than the Hammer Bros. They aren't that bad when you're Fire Mario or when they appear on a platform above you, but when you're small and they appear on the ground in front of you, or even worse, on stairs (shudder...), they're extremely tricky to get around, let alone actually kill, without dying. (This mainly concerns their appearances in SMB 1 and the Japanese SMB 2; in later games they're significantly easier to deal with.) They will throw hammers at random intervals (trying to jump onto or over them? Hope they don't decide to throw a hammer while you do it), and jump, also at random intervals (giving you a split second to react and run under them, unless you've already committed to jumping over them in which case you're dead). They weren't even in Super Mario World, the Amazing Flying Hammer Brothers taking their place, which threw hammers much faster than their ground-based counterparts, making jumping on them a high-risk maneuver, while the Sumo Brothers actually caused lightning and fire to rain down upon you, couldn't be jumped on and were immune to fireballs. In Super Mario Galaxy 2, they became even worse, thanks to the Grandmaster Galaxy. (Not to mention The Perfect Run...)
Super Mario Bros. 3, which made Hammer Bros. easier to deal with, ended up with its own example in the form of Boss Bass, a huge fish that would jump out of the water and swallow you whole for a One-Hit Kill, no matter what powerups you were using, and usually appears on levels where the water rises and falls. He could be killed (usually with a block; blocks that can be picked up and thrown often appear in the same levels), but would come back after only, say, ten seconds.
The Ninja class enemies in Super Paper Mario (Ninjoe, Ninjohn, and Ninjerry). They have highly erratic movement patterns (which includes jumping), are quick, and can throw shurikens in any direction with next to no warning. And when you finally manage to land an attack on them? They love to disappear, leave behind bombs, and re-appear after the explosion. Unscathed. It is no exaggeration to say that the rooms in either Pit of 100 Trials containing these guys are harder than the rest of them combined.
From this hack of Super Mario World played by raocow, the Koopa Elites. The green ones jump, the red ones shoot fire, the blue ones throw their shells into the air at you and the yellow ones do all that and turn into super fast invincible shells. Then there's the Para versions, which DIVE BOMB Mario and charge all through the air while doing the above.
The Lakitus from SMB 1 are worthy of mention. Not only are they Mook Makers, they tend to be very difficult to kill (they fly around quickly at the top of the screen, which means that fireballs are useless and only a very well-placed Goomba Stomp to the head can kill them)—and on some stages, flat-out impossible to kill, since the Malevolent Architecture won't allow for Mario to attain the necessary jump height to hit them. Oh, and they chase Mario throughout the stage. It should say something that Nintendo actually removed these guys from the enemy line-up for many games.
Rip Van Fish in Super Mario World will gladly chase you everywhere and they get quite dangerous in groups or when met with other fish enemies.
Those ^$* %&#@* $ flying Cheep Cheeps, also in SMB 1 and the Lost Levels. There's an infinite number of them, they jump around quickly, usually 3 at a time, and you can't tell where they come out until they already do and hit you. Oh you'll just take take the high ground? Guess what? You need to climb to get to the high ground to begin with. You're probably low enough that you can't hope to react, and since their movement is erratic, you probably will get hit if you try being smart and jump around. Oh, and if you're dealing with them, you're probably having to deal with platforming that expect coordinated jumping. Hooray. But don't worry, you won't see any until you're all the way to World... 2-3?! And that level has way more bottomless pits than any level before it, and no high ground.
The squid in the underwater levels of the first three games are the most dangerous underwater enemy in the first two. They will move down slowly until parallel with you, and then lunge at an upward diagonal. There are ways to get it to go the direction you want it to go, but it can very easily trap you, or run 50 seconds off the clock as you try to find a way past one. Then in the third game, it releases tiny squids that hinder your swimming in Forced Scrolling levels.
Super Mario Bros. 2 has the Blue (Pink in the NES version) Panser. Most of the Enemies in Mario 2 haven't been very imposing. Not even the Red and Green-colored Pansers, who were nice and only stood in one place and shot fireballs at you. That is until you've run into these guys. Blue Pansers are not only able to shoot fireballs in an arc like their Red cousins, but they move as well, and they are very aggressive. These guys without a doubt are the most dangerous enemies in the game, and are sure to cost you a hit point or two if you are not careful.
Banjo-Tooie's Grunty Industries with its MOTHER FARKING TINTOP DRONES. Little mechanical buggers that shock you when they get close and are invincible except for the single second they open their lids every so often. Made even worse by the fact that you can tell if you'll have to deal with them because their corresponding Mook Maker is hard to miss in the rooms, and while you can stop them by shooting out the camera that releases them, some of those cameras are so hard to find or even hit that it will often be better to just make a mad dash for the next door than to waste your Grenade Eggs trying to take them out. And they come in droves. Not groups, droves.
In the first game, Sir Slush, the snowmen from Freezeezy Peak and the winter portion of Click Clock Wood. While they can't move, they really don't need to. They throw homing snowballs at you as long as you're within range, meaning you can't stop for a second, although they'll probably still hit you. They can only be killed with Kazooie's dive bombing attack, which not only works off the game's wonky flying controls but also has a really good chance of not only missing, but sending you crashing into the ground for huge damage. To get one Jiggy, you have to kill all of them in the level.
Banjo-Tooie has the lava hands from the fire side of Hailfire Peaks. They pop out of fiery little crevasses in the walls, which doesn't sound so bad, but you'll unfortunately have to deal with lines of about three or four of them pretty often. They can only be conveniently killed by Grenade Eggs and Ice Eggs, but unfortunately, they don't spawn until you get very close to them, close enough that the former eggs will damage you with its explosion, but the safe line of their spawning is so thin they might hit you anyway. The only other option is just jumping over them, which some of the surrounding architecture thankfully allows you to do. Killing these things gets really tedious since the enemies in this game respawn after about ten seconds.
The Time Trax video game had, well, spiders that would lurk sinisterly in the foreground before pouncing on the playing field. There was no way to tell when they would, it was random, and they were hard to avoid or kill.
Every single Mega Man game had a type of bouncing, spring-like enemy that not only had a ton of HP, but also drained one-fourth of your life bar IN ONE HIT.
The worst of of the bunch must be Sniper Armor from Mega Man 2. Much like the other big mid-bosses in the game, it comes with a whopping 20 hit points. It is too large to jump, but itself can do long and low jumps that take 1/3rd of your health from collision damage! Once you take out the armor itself, there's still the regular Sniper Joe left to deal with which has another 10 HP. All in all, this tough as nails mid-boss comes with the same amount of health like a Robot Master. And to top it off, they're everywhere! Unlike other mid-bosses, you can encounter them in packs of two to three armors in the same corridor! ...oh and don't even think about using the Metal Blade on them.
Worse yet, don't ever walk backwards after killing one, they instantly reappear once the screen scrolls back!
The closest thing to this trope that the Kirby series will give us is probably the Scarfy. At first it merely hovers around innocently - and with that adorable face, what harm could it possibly do? However, if you try to inhale it (like for any other common enemy) it will completely resist it and instead transform. Its face becomes horrifying and cyclopean, and instead of unassumingly flying in place it will psychotically charge straight for you, and even explode at the end of its attack. They aren't too hard to dispatch with exhaled items or Kirby's powers, but in the absence of either, they can be quite scary to confront.
Super Meat Boy is supposed to be Nintendo Hard. But one enemy in the game sticks in the mind of most players, first encountered in the level called Abbadon. It's known by some fans as Demon Maw, Munching Missile, and other names. The first part is that they come from a small black portal and make a Scare Chord-like noise when fired. The Demon Maws are pretty big (about the size of 3 Meat Boys standing next to each other), have homing capabilities, and a moderately tight turning radius. Their hitbox makes this part less threatening than you'd expect, but what really makes a Demon Maw a Demonic Spider is that once it hits something, it explodes and splits into 6 tiny Demon Maws, all flying in different directions. Due to its high flying speed, you'll probably be near it when it explodes, and thus have literally no room to manoeuvre between the mini-maws. Appropriately enough, this enemy is first seen in the fourth world, which is titled "Hell". And Hell isn't even the hardest world! Wait until you get to Rapture, The End, or *shudder* Cotton Alley... and don't get us started on the Dark World.
The original Donkey Kong Country, to begin with a tough game in a tough series, features the walking nightmare that are Rock Krocs from "Stop and Go Station". They only appear in one stage and, unlike most other Kremling mooks, never reappear at any later point in the series, and with good reason: they're totally invincible, extremely well-camouflaged, have glowing red eyes that glare at you in the darkness, and when you hear 'em coming... ugh. You can temporarily stun them by turning the green lights red by hitting specific switches throughout the stage, but this never lasts longer than, say, five seconds, and the final stretch of the stage is crawling with them. The stage is so tough and so early in the game, Rare actually included an easy cheat that allows you to skip it outright (by walking left instead of right at the start of the level). You'll miss out on a ton of collectibles, but if you're a kid playing this shit in the nineties, you're going to want to take it.
Not quite as bad, but still damn difficult, are the Grey Krushas. They are basically stronger versions of the normal Blue Krusha. The only way to kill them is with a barrel, and if you miss you're screwed. Especially considering the nature of the level (its the last actual level in the game). Unlike the Blue Krushas, Donkey Kong's hand slaps won't even yield a banana from them.
And Then there are the Brown Gnawties riding millstones. They are unkillable, even with TNT Barrels, they are large and hard to avoid, and in one level, they actively follow you
In Donkey Kong Country 2 there are kremling enemies that hide in barrels and spring out at you when you are near, not really doing damage but pushing you, often off of cliffs. These in themselves aren't that bad and fairly easy to kill, but they come in different colors. The green kind simply pushes. The yellow kind steals your bananas with each bounce, making him a greater annoyance. The black kind... He doesn't kill you, he just steals those extra lives you're not using at the moment. Not only that, but he tends to only spawn in areas with very low ceilings, and since the only way to stun him is to jump on him, in these situations the only fallback is RUN.
In Pac-Man World 2, when you're treking through the snowy mountains, Rev-Roll Rams will charge into you, usally knocking onto slippery ice or into a bottomless pit. And even if you avoid their charge, they still have the tendency to travel a very far way. The only way to kill them is a two-step process- rev-rolling into them and butt-bounce on their head, which leaves the player open and takes time, and they love to recover before you can deliver the second attack on them. Nasty things.
In the retro-styled freeware game Don't Look Back by Terry Cavanagh, the player must descend into Hell and reclaim his recently deceased wife's soul. On the way there, the player must go through snakes, bats, spiders and other enemies, all of which qualify for this trope (Spiders moreso considering they're actually in Hell, making them quite possibly, real demonic spiders.).
City Connection has that damn smiling, flag-waving Cat, who frequently shows up from nowhere and makes the player lose a life if it is run over. When it appears, chances are the player won't notice quickly enough or is landing from a jump and so can't dodge it. The Cat wants to be hit.
Sonic Adventure 2 has Artificial Chaos. When they are not attacking the player with Combat Tentacles that are hard to predict and can stretch a yard away or shooting laser beams which are also hard to dodge, they are hiding its head/weak point inside its gelatinous body which causes any attack you hit it with to bounce off uselessly. This includes Sonic or Shadow whenever they try to do a homing attack too late, which is particularly notable if you're in the middle of a chain of homing attacks over a Bottomless Pit.
The Red Archer Fairies in Koumajou Densetsu II's Phantasm stage will make you tear your hair out. How do we hate thee? Let us count the ways: They are very accurate with their arrows, those arrows hurt, and they can keep up quite the rate of fire. They appear in the most annoying places, and usually attack in concert with other enemies whose attacks you also want to avoid, they have this habit of jumping away if you want to kill them with your sword or with a subweapon, and worst of all, they are completely immune to Sakuya's time-stopping ability. Yeah. Have fun.
The Combat Tentacles in Ecco The Dolphin. The're large, bodiless tentacles that stick out of walls and grab you and refuse to let go for anything. You can escape with enough charging and fast swimming, but good luck keeping your breath from running out while you do it. And if you're in debug menu invincibility mode, your only choice may be to reset the game entirely or turn off unlimited life so you can die.
Crabs, King Crabs, Trilobites... Anything that jumps out at you from off screen.
The Vortex. Fine, they're easy to shoot out, but they take up to 3 hits - 1 to kill the body, 1 or to 2 to kill the head. Once the heads are on top of you, they quickly take away health at a speed that can only be described as eating you, killing you in maybe 2 seconds if not instantly. On a long fixed scrolling level too.
Distorted Travesty has the Shrouds, which start appearing halfway through the game. Agents of The Darkness that hunt down its enemies, they randomly appear in any level you visit, in some cases even following you from room to room. They always appear in pairs, have enormous amounts of health, their attacks are tough to dodge, their elemental weaknesses change depending on whether they are flying or walking, and their behavior is set up so that they support each other in combat. And if you happen to encounter them while in the midst of a Platform Hell sequence... well, there's always Rave Mode.
Bear Sailor isn't too tough nor does he have some impressive range, but woe is you if you step into this range. To clarify, if they're too close to Claw, they will strangle him. Damage over time created by this is half of his health. Half. It's relatively easy to get out of the grip, but they can repeat it just a second later.
Red Tail Pirate isn't too much of a challenge in seventh level when he first appears, but then there are Shipyards and Tiger Islands where he parries pistol shots and sword attacks with near-perfect ratio and making fast counters against your attacks. On smaller ledges, you will get hit at least once.
Peg Leg is fragile and more of an annoyance than a real threat in ninth level. Then, level later, he gets better. Now, his combo is two-shots, one high and one low, making it difficult to avoid if you don't pay enough attention. And given plenty of traps trying to kill you in the meantime, you are likely to lose your focus and get stun-locked into death. They also have invinsibility frames sometimes which can trick you into attacking them which will almost certainly cost you another 20 hit points.
Nothing beats Tiger Guards though. Tough, agile, dealing ridiculous damage, immune to pistol and dynamite, often paired up with traps and Chameleons... Thankfully they appear only in the final level.
Crescent Pale Mist has areas in certain places of each Chapter of the game teeming enemies waiting destroy Yunou almost impossible to avoid attacks that can potentially deal tons of damage in a matter of seconds, especially if there's a Boss in Mook Clothing among bunch.
Captain Comic I has the Killer Bees, found in The Cave, The Shed and The Castle. Like everything else in the game, they do two points of damage, but the way they move entirely in horizontal and vertical lines makes them extremely good at creeping up on you to attack from a place where you can't shoot them, usually either your knees when you're on a ledge, or from directly overhead. So effective are they at stinging the player to death that players getting paranoid and messing up jumps results in almost as many deaths as direct stings do. Perhaps their one mercy is that like all other enemies in this game, hurting you also destroys them.
RosenkreuzStilette featured several gargoyle statues that had a lot of HP and could also take off one fourth of your life bar in one hit. Since this is a Mega Man clone, they behave the same way as the spring enemies mentioned in the Mega Man section above.
Nebulus has an enemy that resembles a giant floating clump of marbles. This enemy cannot be killed. Worse, it doesn't move around the tower — it moves across the screen on a tangent perpendicular to the tower. If it's coming at the player from the left, no matter how far around the tower the player goes it will still be approaching the player from the left.
Sonic 3 & Knuckles features the Orbinauts in Launch Base Zone. These are round, vulnerable balls with four damaging spikes orbiting them. Unlike the icy versions in the previous level, these are smart enough to keep the spiked balls instead of throwing them out and leaving themselves defenceless. The only way to hit them without getting harmed yourself is to use the insta-shield (which is very risky). And if you are playing as Tails or Knuckles, you don't have any way to hit them w/o getting hit!
Nearly anything can qualify in Heart of Darkness, but the shadow spiders are a perfect match. Your character usually fight them while climbing, limiting your movements; they drip poison on the handholds that can make you fall, and they evade any shot you can fire at them unless you let them come very close. Then you have a split-second to blast one before it'd eat you. The one end of a level where a literal swarm of shadow spiders is attacking you is, thankfully, just the game giving you the hint that you should flee, and fast.
Band Land's grasshoppers in the original Rayman. They're extremely good at predicting your moves, and will dodge and block most of your attacks, especially if your fist isn't fully upgraded. They also have a tendency to rush you and attack you repeatedly the instant you turn your back. And this is in the second world. The first game is known for its Difficulty Spike for a reason.
Symbiotes in the PS1Spider-Man game. They take a ton of hits to kill and dish out a lot of damage, along with being able to jump around and fly up to the ceiling, along with hitting you from a distance with whip-like tentacles. The Web Trap and Web Ball moves are pretty much ineffective against them as well (though the former can at least stun them for a brief second.) Just one can give the player some serious trouble, and eventually the game expects the player to fight 2 or 3 at a time. The only saving grace is that the game will occasionally give the player Explosive Barrels and magnesium power-ups that cause Spidey's web to catch fire, and since Symbiotes are weak to fire, a single barrel or flaming Web Ball will One-Hit Kill them (though thanks to somewhat iffy controls, the Web Ball can be hard to pull off on command.)