We were probably thinking the exact same thing as you: how the hell did George Clooney come to appear on South Park? It seems like such an odd fit, and if anything, the show's creators, Trey Parker and Matt Stone make fun of people in Hollywood like Clooney. As it turns out, Clooney was an early adopter of South Park and spread the word about the show, helping it to become popular back in its beginning.
He first appeared on the show as Sparky, a gay dog who takes refuge at Big Gay Al's Big Gay Animal Sanctuary. A year after his debut role as Sparky, Clooney was back. This time as Dr. Doctor in the South Park film, South Park: Bigger, Longer, and Uncut. We wonder if he'll make another appearance in one of Parker and Stone's projects in the future.