Poison Headcrabs – Half-Life 2
Half-Life’s headcrabs were one of the scariest things I’d ever encountered in a video game when I first saw them, and the poison variant in Half-Life 2 is just worse all around. They don’t kill you outright, but no matter how much health you have, getting bitten by one reduces you to 1 HP, which means you’re a sitting duck for any other bad guys around. That’s annoying enough, but they’re also small and horrible, and the game likes sending them after you in dark, scary, cramped environments.