Boos are spectral beings with beady eyes, long tongues, and sharp fangs. The first appearance of Boos was in Super Mario Bros. 3, where they appear under the name Boo Diddly, a pun of Bo Diddley. In Super Mario World, there are much larger variations of the Boo under the name of Big Boo. All Boos are ghostly creatures that are able to pass through solid matter.