Boos are enemies in the Mario series that are supposed to be ghosts of dead people. Boos first appeared in Super Mario Bros. 3, and was a threatening enemy. In the Luigi's Mansion series, Boos were inspired to make other types of ghosts. Boos were even added to the game!
Even though Boos seem to be not fitted with anything else, they can. In Super Mario Galaxy, you can find many types if boos. For example, Octoboos resemble normal Boos, only with different behavior and look. Boos are very shy. If you take one look at them, they will cover their eyes and stop trying to kill you. But when you turn away, they open their eyes and continue to try to kill you. Boos are probably one of the most menacing enemies ever! Their 4D counterpart is the Vampire Boo.