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