Oth

Every now and then, in books and movies and other fictions by Humans, aliens from space try to take over the Earth. They generally fail, which is kind of dissappointing. So I thought of an alien we would not be able to stop.

There are an infinite number of possible universes, so there must be one that contains whatever we can think of, including The Oth. The Oth lives in a universe that doesn't know about us yet, but when they do, they will invade and they will succeed. Because they are smarter than we are. And have more arms, wings, beaks, and skis than we do.