This is probably the only animal type villain that I'm including on here. It's also one of the most terrifying fucking things that has ever graced the screen. Just learning about how they reproduce is enough to make you squirm. If you ever want to aptly describe how bad you felt when you had the stomach flu, just say that you were reenacting the birth scene from the first Alien movie, and people will almost start vomitting themselves. So let's do a rundown of this creature. It's a beast that hunts solely on scent and sound, being that it has no eyes, so there's really no way to hide from it, it'll eventually find you. It learns quickly on how to get into secure locations, it has two mouths to bite you with and it bleeds fucking acid. As if the rest of it wasn't terrifying enough, you have to add the fact that it bleeds acid. So basically, if it gets anywhere near you, you're screwed. Either he's going to chew your face off, or you are going to shoot it in the face, and the acid will chew your face off.