The Valley of Gwangi (1968)

In a small Mexican town, horse trader Tuck Kirby meets up with old flame T J Breckinridge, owner of a rundown circus. He runs into palaeontologist Professor Horace Bromley, who has found a living eohippus, the prehistoric ancestor of the horse. Nursing their own agendas, the scientist and the cowboy venture into the Forbidden Valley only to find that larger, more deadly creatures have survived from prehistoric times.