Because Ruffian was such a large filly, larger even than many colts (including Foolish Pleasure), they used geldings to portray her in the film.
Sam Shepard so completely resembled Ruffian's trainer Frank Whitely, He once mistook vintage footage of Ruffian with Frank Whitely as a daily shot when viewing editing of the film.
A prosthetic leg with a kicking lever is used in the scene where Ruffian is shown repeatedly kicking a wall as a veterinarian attempts to set a cast on her injured leg.
In the scene where Ruffian breaks down during the Great Match Race with the colt Foolish Pleasure, the character of journalist Bill Nack is seen running onto the track attempting to help Ruffian. He very narrowly avoids colliding with Foolish Pleasure. This actually happened. Bill Nack later described going under a barrier and running onto the track with his eyes fixed on the injured filly Ruffian. Only when the jockey of Foolish Pleasure screamed, "Hey!" to get his attention, did Nack realize he was directly in the path of the horse. Bill Nack stopped in his tracks, and narrowly missed being run over by the 1,200 colt, who was finishing the race.