Half-Life/Enemies

Barnacle
Barnacles are stationary aliens that attach to ceilings. They have extremely long and sticky tongues that they let hang down from where they hide; if something gets stuck to the tongue, they immediately begin reeling it in, hoping to get another meal.

Headcrab
Headcrabs are your basic alien enemy. They're weak, but also very small, making them hard to hit. Headcrabs are usually found hiding in damp or dark areas, waiting for prey to ambush. When attacking, they launch themselves at their intended victim's head, making a screeching sound. While this would cause trouble for most other humans, Gordon's HEV suit protects his head.

Zombie
Zombies are humans that have fallen victim to headcrabs. The infestation has mutated their hands into long, sharp claws, and there is a giant gaping hole in their torsos. Zombies are usually found standing around doing nothing, but are sometimes found on the floor, feigning death until they are shot or someone approaches. To attack, zombies slash with their claws.

Gonarch
A giant beast that looks similar to a spider, Gonarch is a four legged creature with a large sack (similar to an abdomen) that hangs down below where its legs meet. As the second to last boss, Gonarch can only be found once during the game and it takes a large amount of damage to kill. The sack is Gonarch's weak point, and it can only be harmed when Gonarch isn't in "cinematic mode" - in which the beast progresses farther into its lair between battles.

Nihilanth
The boss of the game.