Not really an enemy per se, but The Darkness had a sequence where you go through a hellish vision of WW1. There were zombie germans there, that were pretty messed up, but more creepy for me were the NPCs. There were weird patchwork British soldiers, sewn together from bits of other men. One guy in a trench would shoot himself in the head, only to come back to life and do it all again. A basically skeletal corpse sat in a sewer, playing a harmonica. A field hospital where men with no limbs, no eyes, no faces etc. sit there begging for their mothers. You talk to a man with no limbs and all his face blown away but his mouth, and he says something about 'keep a stiff upper lip' before sobbing and calling for a nurse, or his mother, or anyone. And when you went inside a massive railway gun to fire it, there was this one creepy German dude sitting behind you. You walk in, and all you hear is this deep, menacing voice. You turn around, and there's a limbless zombie German talking to you. Creepy stuff. Damn, that entire part of the game was creepy as all hell.