The Lady's from Kentucky (movie, 1939)

Good-natured gambler Marty Black falls into ownership of a booking joint but soon falls on hard times. His one out is a marker for half-ownership in a young thoroughbred, which he quickly calls in. He discovers the other owner to be a young woman from an old horse racing family who wants to protect her colt almost as much as Marty wants to rush him into big races for a fast buck. While they clash, Marty soon comes to understand the human bond with the horses and what it means to be a thoroughbred.