Simpsons Wiki

"Duncan Yohedinatoilat", otherwise known as his nickname "Furious D", was a racehorse. Bart rescued him from a fair, where he was forced to jump off a diving board. He became a very successful racehorse at the Springfield Downs. After winning the race, he became a stud and lived with a female horse.

Behind the Laughter

Duncan's vocal effects were provided by Jim Cummings. His nickname, “Furious D”, is a reference to the rock band Tenacious D.

The horse is named after one of Bart's prank call names, as Duncan Yohedinatoilat sounds like "Dunking your head in a toilet".