Mustang! (1959)

In the hills and forest surrounding a horse-breeding ranch an unbroken wild mustang (played by a male horse named Autumn Moon, who is not a human actress) who has been a constant challenge to the ranch owner (Milt Swift) and his hired hand Lou (Steve Keyes) who have attempted, and failed, to kill the horse several times as a bad influence on the tame horses of the ranch. One of the cowhands, Gabe (Jack Beutel)and the owner's sister, Nancy (Madalyn Trahey), believe that the mustang can be captured and tamed and be made a valuable asset to the ranch. Despite opposition by Lou, they set out to do so.