Hmm, lots of them.
The Hellknight from Doom 3
The Archvile from Doom 1/2
The headcrabs from HL
The fast zombies in HL2
The Sirian Warbulls from Serious Sam: SE
Hienrich in RtCW
The Progenitor Seekers from Homeworld 2
The guys with MG42's in CoD and MOH series (they ARE godforsaken monsters, all right!)
The Undead Horde in the last level in W3:RoC
The Xenomorph Queen in AvP2
The Parasite Dog from Quake II
Lucifer from Painkiller
Slezz from BloodRayne 2
The Ninja jumping jackrabbit girls from Prince of Persia: Warrior Within
and a heckuva lot more. Pain, pain, pain, everytime I encounter them.