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Duncan, also known by his nickname Furious D, is a racehorse.
Bart rescued him from a fair, where he was forced to jump off a diving board.
Bart and Homer then trained him as a racehorse, building his confidence and giving him the persona of "Furious D". With Bart as the jockey, he became very successful, winning a large number of races at the Springfield Downs.
After winning the championship race, Duncan retired to live out his life as a stud.
 Behind the Laughter
Duncan's vocal effects were provided by Jim Cummings.