Great Dan Patch, The (1949)
Starring: Dennis O'Keefe, Gail Russell
Plot: Chemist David Palmer comes back to his hometown in Indiana to get married to his girlfriend Ruth, a schoolteacher. He finds his father excited about the harness racing potential of a new mare that he's just traded for. When the mare is injured while winning her first race, David's father uses her for breeding, and she has a colt named Dan Patch, who proves exceptionally fast. Meanwhile, Ruth becomes increasingly frustrated with David's continued interest in horses, and with his friendship with his father's trainer and the trainer's daughter.