"Oh man, I totally hate those guys!"

[MissDumpling]

Fucking MURLOCS and their annoying sounds.
A close second is the Slime thingys in the same game with Lousy drops and tendencies to spawn a clone.

 

[Anarchemitis]

Black headcrabs from HL2 and the spiders / zombies from Dark Messiah of Might & Magic. All three have an ability to drop your health to 1 instantly, which, imho, is the most frustrating implementation of poison I've ever seen.
HL2 at least used that with some moderation, so it was okay, but DMoMM sometimes got plain frustrating because of that.

Any enemy with an overly potent stun ability in any genre. Nothing is more infuriating than being beaten up by some weaklings because some Medusa / spellcaster / whatnot baddie stunned you for 10 seconds.


Oh, and I absolutely love zombies, normally, except for DMoMM

 

[ar4yyyy]

The splicers in Bioshock...creepy

 

[Jovlo]

Yup, the Redead from the Zelda series...
It's the way they just stand there moaning softly.
Then when you get noticed they slowly, oh so slowly come for you.
When they (or you) finally get close enough they unleash this soulrending shriek that makes you freeze and then they come for you fast while you can't do anything.
They really freaked me out as a kid.

The Redead from the Windwaker were less scary but when they died, it took a minute or so before their bodies would disappear.
Always thought they would get back up in the beginning.
Funny detail, if you shot them with your bow, the arrow would get stuck in them with a satisfying 'thud', doing no damage. You could turn these guys into porcupines before you killed them.

 

[Conqueror Kenny]

Anything that flys when you only have a melee weapon. That's the definition of unfair

 

[ThaBenMan]

As for me, zombies.... I want to kill the person who gave them this idea.

But then they would probably just turn into a zombie and then you'd be screwed :/

The most horrifying zombies I've ever dealt with were probably the ones in Thief: The Dark Project. My god, the sounds they would make. And you felt so helpless because you could only kill them with holy water.

 

[MichaelH]

Old-school -- the bulletproof cars in the Spy Hunter arcade game. I could never get the hang of ramming without going off the road and dying. Consequently, every Spy Hunter game I've ever played has lasted 1 minute and 20 seconds. I hear there's a boat level...

 

[Kaos Incarnate]

The jellyfish from The World Ends with You, mostly because I button-mash for top screen and focus on the bottom screen most of the time. You need to be competent on the top screen to consistently get Fusions, which eventually becomes the only way to beat these guys when they split into like 8-9 copies, and then those copies start splitting...*shudder*

I'm fine with them.

 

[Kayevcee]

System Shock 2. Psi reavers. Hideous beyond words, incredibly damaging and they keep coming back until you find the brain and exploderise it. Every encounter is an adrenaline trip, and every second after you take the first projection out but before you've found the brain is like three lifetimes.

-Nick

 

[Writerling]

Trolls in Oblivion 'cause they jump out at you from freakin' nowhere and scream their lungs off at yoi.

 

[TheMadDoctorsCat]

System Shock 2. Psi reavers. Hideous beyond words, incredibly damaging and they keep coming back until you find the brain and exploderise it. Every encounter is an adrenaline trip, and every second after you take the first projection out but before you've found the brain is like three lifetimes.

-Nick

Hell yeah. And what about the first psi-monkey you meet which throws exploding fireballs instead of regular psi-blast? I was like, "Haha, monkey, no biggie, can deal... WHAT THE HELL WAS THAT??!!!"

 

[Librarian Mike]

I loathe the Medusa Heads in Castlevania 4. Seriously, graze them when you're walking through the level and the send you flying backwards into the bottomless pit of despair.

Oh, I totally agree with that. You know how it is, your shoulders just involuntarily drop when they start moving towards you. Medusa Heads, along with pretty much everything else in Castlevania 3 are evil.

 

[The_root_of_all_evil]

Warning : Nervous Dispositions should not click the spoiler.

I really mean this.

Spoiler

 

[the monopoly guy]

what the hell...is that?...MAKE IT GO AWAY
don't forget those little mushroom-I can't remember the name-guys in mario "sorry but your princess is in another castle" I don't care! I just kicked some giant turle that breaths fire and has spikes in his back jsut for some ditz who doesn't have the common sense to lock her bloody door!

 

[teh_gunslinger]

Oh, just remembered:
Alpha Centauri. Those orange bastards. "Children of God" or whatever their name was. Kept blabbering about God and my sins and whatnot. Sins my ass. See this Planet Buster?

EDIT:
Hmm. Think it was "The Lords Believers". Anyway the leader was Sister Miriam I think. Ahh, good times with nukes.

 

[The Potato Lord]

Rabid dogs,Bullfango(Monster hunter),Anyone who expolodes when killed

 

[TheKnifeJuggler]

Anything large, moving and evil in Katamari Damacy.
Especially cats four times as big as you.