The Bats from Parasite Eve 2. One hit-point, any attack aimed at them kills them... But let them touch you and you are paralyzed for 5 to 8 seconds while it slowly seeps your health point per point... And they come in swarms of ten or more. Preferably at arm's reach. The precognition REALLY applies to this one.
Oddly enough, they're instantly killed with a flashlight attachment.
Also, the Knight/Bishop GOLEM. These cyborgs are not only powerful, but can turn invisible. In fact, they automatically land a hit on you when you first "detect" them, grabbing you from behind without warning and either poisoning you or paralyzing you unless you can kick them away in time. In addition, they use a decoy attack. Fall for it and it guarantees a hit. So not only are you fighting an invisible enemy that swipes at you with the power of a kodiak bear, but you start the fight with a quarter of your health gone and a serious status ailment. What is worse is, while they only show up in the regular game mode towards the end of it, these bastards show up way earlier in the Nightmare and Bounty modes. And since enemies hit for more damage in these modes, if they land two hits on you it's Game Over.
Dead Space has a number of enemies that can prove frustrating, but the Guardian and the Crawler stand out. The former spits out "pods" that explode with considerable force (though, thankfully, not as much as an Exploder) if Isaac gets too close and which otherwise sprout a quill-throwing tentacle and fire at a much faster rate than Lurkers do—and the only way to kill the pod is to sever the tentacle. The Guardian spits out pods at a fairly steady rate, and the only way to kill it is to sever the five tentacles that it attaches to the wall—and which it can pull inside of itself and reattach if it feels like it. Oh, and did I mention that getting too close results in an Instant Death cutscene? Crawlers are small, tend to come in groups, move quickly and can be hard to hit, especially due to their leaping around. Dividers are actually easier to handle then Crawlers; hit the Divider with Stasis, then calmly blast each Crawler component. The Twitcher (due to its speed, random movement and ability to "jink") and the various Advanced Necromorphs, particularly the Scorpion and Lurker, get honorable mentions.
It should also be noted that the whole while, the armless upper torso that is the core of the Guardian is letting out screams of apparent agony the whole while. Not only is it really disturbing, but can prove to be a major distraction while fighting the pods and the Guardian itself.
Swarmers are like ultra-tiny crawlers but ten times as dangerous: They come in gargantuan groups, they stop you dead in your track until you shake them off (and each individual one takes a second or so to shake off, while a dozen more has jumped on you in the meanwhile), and can with their cumulative damage kill Isaac in a shorter period of time than any other enemy in the entire game except for the Guardians. And did we mention that only two weapons are anywhere near useful against them, and one of them is useless for everything else, and the other one you get fairly late in the game?
Don't forget The Pack in Dead Space 2, which can be incredibly irritating on harder difficulties, and Stalkers, which are incredibly smart, as well as fast. Stalkers are capable of playing hide-and-seek, something no other Necomorph can do, and their presence is enough to make anyone's paranoia level go through the roof.
The Leapers get an upgrade in the sequel, now they're fast as hell and kill you dead before you can even finish your girly scream after the thing drops on you from the other side of the map.
Did you like the regenerating enemies in 1 and 2 that couldn't be killed by normal means and hounded you for entire levels? How about a dozen? Dead Space 3 is happy to provide!
The standard feeder is a Fragile SpeedsterCannon Fodder who you'll share a love-hate relationship, because one intense firefight will leave you with an armory's worth of ammo and supplies. However, the Enhanced Versions are complete hellspawn. They're tougher than the standard Slasher and still come in packs!
Unitologist Rocket Soldiers always enjoy ruining your day from a far away location with lots of explosions. They're not much tougher than other human opponents, but they always show up when you're focusing your attention on the monsters currently eating your face.
Festers in The Suffering are bloated corpses that periodically tear open their stomachs to release a swarm of giant rats, which explode when they reach you. These rats home in on you, and if one hits you, you'll likely be stunlocked until the entire swarm explodes in your face. (It's possible to shoot the rats, but you'll never get them all in time unless you rapidly equip the tommy-gun, which isn't very good against the Festers themselves.) Axe attacks hurt the Festers quite a bit, potentially allowing you to finish them off before they can release their rats, but their melee attack is quite powerful and does damage in a circle around them, so it's recommended you keep your distance. And as a final fuck-you, killing a Fester with a non-fire attack will automatically release a swarm of rats as it dies, and fire weapons are in short supply in this game.
In a game already the stuff of nightmares with its surprise random encounters which can mystically appear in a room you've been through several times already and often appearing with little or no warning directly in front of you (screaming ensues, if the player is particularly into the game), Fatal Frame has its own demonic spider to fear and fear greatly. The Blind Ghosts, especially in one encounter in a cramped corridor, can turn fear and terror into fear and loathing. They can pass through walls (wherein they become difficult to detect and invincible), move much faster than your character, lunge at you when provoked or if you move (being blind of course means they have superhuman hearing), may not flinch when attacked (this counts as provoking them, naturally), can teleport behind you at any time (including when they're invisible because they're eight feet inside a solid wall, leading to more "AGHHH!" moments when you suddenly catch sight of them less than a foot from your face), and have a nasty tendency to kill you in a single hit. And did I mention they like to counterattack?
Therazinos from Dino Crisis are not only insanely strong, highly dangerous and -thanks to their traveling in pairs- nearly impossible to bypass, but they're also given their own Press X to Not Die cutscene which makes you think they're a one-off Dual Boss. Nope. They're a regular enemy from that point on! Be warned: save your poison darts or get ready to snap a few controllers dealing with them.
The Oviraptors from Dino Crisis 2 are very troublesome, to say the least: they're very small and extremely fast, hunt in rather large packs, and are nearly impossible to target all at once as they have the tendency to surround you completely. Worse, they have a ranged attack so while you're taking out the ones in front of you, the others can hang back and spew acid/venom at you. They also like to jump-kick you from out of nowhere, have a high respawning rate...even if you return to an area you just left a second ago, and will jump onto you to peck you to death.
The Shibito in Siren are giggling, bloody-eyed Japanese farmer zombies who somehow manage to pop you with a headshot from across the map through a copse of trees if you make even the slightest error in traversing the level. Apparently becoming Shibito imbues one with the accuracy of a Navy Seal sniper. There's some good news, though—you can actually survive one shot to (hopefully) dive back into cover... and you can use sightjacking to tell exactly where they're looking and when, so you can stay the hell out of the way. Also, not all of them have rifles—although some of the ones that don't are nearly as bad.
For an example of "nearly as bad," look no further than the Crawlers. Their melee attacks can kill you in two good hits, depending on the character—the same as a gunshot, for reference—and they're fast and particularly alert and difficult to hide from. Oh, and some of them can walk on walls. The plus side is that since they move on all fours, they can't open doors... and they're usually found in the presence of a shibito brain, which will knock them all out at once if it's taken out. The bad news there is that shibito brains are Goddamned Bats, due to their tendency to be able to tell you're there without seeing or hearing you, then run away quickly.
Let us not get started on the flying Shibito, who are essentially snipers with wings.
Schisms, Needlers and Swarms from Homecoming, Victims and Wallsnatchers from The Room, and on occasion, Straitjackets from 0rigins. Honorable mention goes to Hanged Scratchers from the original.
The Smogs from the same game. Not only do they have a ton of health, their attacks can only be dodged by rolling away, as they can stun you with a smoke cloud that is unavoidable (and does damage). Also, if you can't break out of it, they knock you down, dealing even more damage. While it's easy to run past them, they are also the only enemies to have a ranged attack, and very damaging one at that. On the bright side, they can be downed in two shots if you wait until they expose their chest cavity to shoot them.
The nurses (particularly in the third game where some have guns), the Abstract Daddy monsters in the dark and narrow hotel corridors in the second game, and the Slurpers (3 again), which can knock you down again immediately after you get back up, repeat until death. Also from 3 are the Pendulums, which are hard to hit, very lethal, and best just avoided.
The Dolls from Silent Hill: Downpour, especially if you're going for a no-kill run (which is required for an achievement). Not only are they invisible without the forensic flashlight, but they appear in huge groups.
The original does boast at least one enemy of this type: The Romper, which appears in the Otherworld town streets later on. Like the Air Screamers, Rompers spawn endlessly. Also quickly. And they can do a lot of damage if you stop to fight them (Especially since stopping means you're soon engaging more than one of them at a time).
Crawlers are this to Cold Fear. It takes a full clip of shotgun shells to kill one, and they are fantastically speedy and stealthy wall masters. They have a nasty habit of jumping on to the ceiling and hiding in a ventilation shaft after you have damaged them, then reemerging from a different ventilation shaft elsewhere in the same room so they can sneak up on you.
Also, their Weaksauce Weakness was supposed to be a fear of bright light, but that really just makes them wander off so they can attack you from behind later.
After its transition to 3D, The zombie MMO Dead Frontier added several "Boss" zombies throughout the maps. Pretty much the only ones of them that aren't an example are the Bones (which are only moderately more powerful than normal zombies) and the Brutes (which are ungodly slow and whose main gimmick, exploding upon death, is easily avoidable with ranged weaponry). The rest? Oy...
The Reapers and Tendrils will oneshot low level players easily with their insane melee attacks, and the Tendrils in particular have a massive attack range.
Sirens don't even have any actual attacks, and yet they are the bane of many players because as soon as they detect you, they emit a scream that alerts pretty much every single zombie in the area to your location, with predictable results.
The Spiders are ungodly fast and nearly completely immune to knockback.
One of the more recent updates added Burning Zombies to the mix. They are as (if not more) fast than the Spiders, always spawn with maximum aggro (which means they home in on you at top speed) and, contrary to what you may expect from a Man on Fire, they have a massive amount of health. On the plus side, they tend to drop mid to high tier loot when they finally go down.
Rule of Rose: Most of the imps count, if nothing else because they usually respawn and/or come in hordes. Some jump onto Jennifer so she can be pummeled by the rest around her. Other types (birds and pigs especially) enjoy knocking Jennifer down... and then doing it again as soon as she stands back up. You'd better start getting Brown searching for some sweets...