Muybridge originally published the images on a cabinet card in his series ''The Horse in Motion'', which featured 5 similar cabinet cards depicting other horses as recorded in motion by Muybridge during June 11-20, 1878. The subtitle/description of this card was "Sallie Gardner, owned by Leland Stanford; ridden by G. Domm, running at a 1.40 gait over the Palo Alto track, 19th June, 1878.'' A retouched version of the card was published in 1879.
Claims that these images enabled Stanford to win a bet (that a galloping horse momentarily has all four feet off the ground) are now considered apocryphal, since this idea was always believed to be true. However, what the images did reveal was a surprise that this occurs when all the legs are bundled up *beneath* the horse, and not when stretched out as had been assumed for paintings of horse races.
This is not a film but a series of photographs.
This movie proved that horses actually lift all four paws off the ground when galloping. In fact, the film was financed by Leland Stanford, industrialist, farmer (he was even a racehorse breeder), governor of California between 1862 and 1863, to clarify this issue of take all paws off the ground simultaneously.