6. Headcrab (Half-Life)
The headcrabs are easily the most recognisable enemy in the Half-Life series, and arguably in the history of FPS games. They are quite obviously derived from the facehugger of the Alien movie franchise, looking like an oversized, fleshy spider that runs around on four legs and is capable of leaping up to around 10 feet into the air. They make screeching noises and primarily attempt to surprise the player; once one dives out of a vent into your face, you'll find yourself cautiously combing the hallways for the next hour of play, only for another to strike just as you've recovered from the heart attack. Like the facehuggers, they attach themselves to a victim's head, though instead use them for mind control, operating the human host as a more imposing foe for Gordon Freeman and other allies. There are also other variations of the headcrab, most notably the Toxic Headcrab, which appears in Half-Life 2 and will reduce you to 1HP after an attack. Needless to say, they are absolutely terrifying. Even though the headcrab can be easily taken out with a mere crowbar, their slipperiness and their repulsive look makes them one of the most fierce enemies in all of gaming.