The Valley of Gwangi (1969) on Netflix

It's the turn of the nineteenth into twentieth century. Breckenridge is a traveling wild west show. In addition to starring in the show as a trick rider, T.J. Breckenridge took over managing it following her father's passing, but it is doing poor business and threatens to close because of it. When the show is in a town south of the Rio Grande, T.J. receives a surprise visit from Tuck Kirby, who used to be with the show and was her former fiancé, until he walked out on her and the show out of fear of commitment. Now a horse trader, Tuck, who is still attracted to T.J., wants to buy the show's star horse for Wild Bill's show, but also to help get T.J. out of a financial hole. But T.J. has a new attraction which she believes will get the show out of trouble: "El Diablo", a miniature horse the size of an average dog. In his recent encounters with a paleontologist named Horace Bromley, Tuck knows that El Diablo is an eohippus, a prehistoric creature long believed to be extinct. Professor Bromley had found a fossilized footprint of an eohippus in the area. The local gypsies, led by Tia Zorina, believe that stealing the creature from a place called the Forbidden Valley, is a curse and must be returned. Knowledge of El Diablo's existence within this small group leads to the different parties trying to get their hands on the creature. But Tuck, his young orphaned local guide Lope, T.J. and her associates, and Professor Bromley discovering the location of the Forbidden Valley changes the focus to what they consider bigger prizes to be found there. However, in wanting to get the biggest prize out of the valley alive for the world to see may open up a proverbial can of worms they may have ultimately wished remained closed.