Colonial Affair, the favorite in the $3 million Breeders' Cup Classic, broke a bone in his right hind leg by kicking the wall of his stall at Churchill Downs, ending his racing career.
The injury, which isn't life-threatening, was announced by Scotty Schulhofer, the trainer of the 4-year-old colt.
Colonial Affair helped make racing history last year when he won the Belmont Stakes with Julie Krone on his back; she was the first woman to ride the winner of any of the Triple Crown races. He also won the Jockey Gold Cup in October.
He headed for Louisville, Ky., as the early favorite in the Breeders' Cup Classic.
"He'd never been better in his life," Schulhofer said. "He loved the track, and he was galloping like a champion. I think he could have won the Classic easily and won the title of Handicap Horse of the Year."
Schulhofer said Colonial Affair got agitated when horses arrived at his barn around 2 a.m. Saturday, and kicked the back wall of his stall. Nothing was noticed until he was getting ready to be taken out for exercise, when a bruise was detected on his right hind leg.