If you have played the Half Life series then you will know that the headcrab can become anything. They might not be big or gross but they offer scary evolution. This was the first creature you encountered following the fateful experiment starting all the way back in the original Half Life. They are relatively easy to kill so you move on in the game. Then they turn to what looks like a man looking disoriented. This is also your enemy and you have to take it down, with more of a challenge. They can also turn into zombies surrounding you with scary moans and cries.