Shukakku is the demon that is in Gaara giving him most of his power AND his insanity. Shukakku is able to fire off sand chunks that look like and act like kunai. In the English dub, Shukakku is made sounding like an insane cowboy.
Shukkaku is the Ichibi, which is the One-Tail Jinchuriki. Shukkaku is very powerful as Gaara can still not fully control him.