George Clooney inquired about guest starring on South Park and was offered the role of Stan's dog. He took it

If you’ve seen basically any episode of South Park, you’ve probably seen an episode that parodied a celebrity. The 200th episode claimed to have made fun of 200 separate celebrities. Some of the more notable ones have included Tom Cruise, Kanye West, and Barbara Streisand. Most of the time, these celebrities are impersonated (obviously).

Some celebrities, however, have come on the show to voice themselves. Jay Leno, Robert Smith, and the entirety of the bands Radiohead and Korn have lent their voices at one time or another. Cheech and Chong even came in for an episode once. 

It was more common during South Park’s earlier seasons for celebrities to come and guest star as characters. Trey Parker and Matt Stone often offered celebrities non speaking roles as a joke, but celebrities often accepted anyway. 

One such case was when George Clooney voiced Stan’s dog Sparky in season 1. All the barks in that episode are from Mr. Clooney himself. Jay Leno also provided the meows for Cartman’s cat at one point. 

(Source)