The horse, trained by Wesley Hawley and ridden by Chris Landeros, fell at the top of the stretch in the day's second race. A pair of horses behind the incident threw their jockeys.
The son of Stay Thirsty, who entered the race with a record of 7-0-1 in 14 career starts, was examined by an expert team of veterinarians who observed a separation of two bones at their joint.
"During their triage and evaluation, they observed a disarticulation of his left front fetlock," Kenneland and Breeders' Cup said in a statement.
"Given the extent of the injury, it was unfortunately determined that humane euthanasia was in the best interest of the horse." (Reporting by Frank Pingue in Toronto, editing by Pritha Sarkar)