One of the more dangerous aliens in the universe, the Xenomorph have a knack for keeping their enemies on their toes. From their larval stage of a Face Hugger, to their more dominating evolutions like the Hunter (pictured above), they are all capable death machines.
And if it wasn't bad enough, if you do manage to fight them off, you can still be damaged as their blood is a highly corrosive acid. They seem not to discriminate between what enemy is what, beside not attacking their own, so everything is considered a target.
While they skitter in the shadowed hallways, Xenomorphs have hunting, hard shells, acid blood, and their pure viciousness on their side that can lead anyone to wet their pants. Unless we're talking about the Alien: Colonial Marines version of them, then just wait 5 seconds and they'll fall through the floor.