Rectuma (2003)

Grossout comedy starring Bill Devlin as Waldo Williams, who on returning from a holiday in Tijuana finds that he has been infected by the Mexican Butthumping Bullfrog, causing him plenty of pain and a whole lot more gas. After visiting a whacky scientist, Waldo finds that his derriere has started glowing green with radiation. It's not long before his rear end takes on a life of its own, detaching itself from the rest of his body to go on a killing spree, leaving tracks wherever it goes and incriminating Waldo in the murders. But being framed as a murderer is the least of Waldo's problems as his rear grows Godzilla-big and wages all out war against Los Angeles...