Ah yes, the Xenomorph, aka the alien, aka the monster that we subconsciously think is going to rip through our chests any time we have a little indigestion. Although we get to see these guys in detail in the sequels, it's Ridley Scott's original film that made these little buggers truly terrifying. You've got the classic "chestburster" scene, but once the Alien grows up, that's when things get really freaky. Molecular acid for blood kind of sucks for anyone trying to attack this thing, but why would you attack a creature that has a tiny little mouth with fangs that shoots out of a bigger mouth with fangs? You wouldn't attack it, that's the answer. Because it attacks you.