Cowboys and dinosaurs.
It doesn't really matter what I say after that. If you want to see a movie with cowboys and dinosaurs, it doesn't matter how good or bad it is... You will want to see The Valley of Gwangi.
That said, it is still an enjoyable slice of Ray Harryhausen's patented Dynamation technique. A wild west show is travelling south of the border when into its possession comes a miraculous miniature horse that shouldn't be: the extinct Eohippus. Unfortunately for the travelling show, a scheming scientist unites with superstitious natives to free the Eohippus, resulting in a chase back to the valley where the evil spirit "Gwangi" lives. The Gwangi, an Allosaurus, is brought back to civilization and... well... the usual ensues.
In fact, for a mid-century film in either genre of Sci-Fi or Westerns, things run pretty typically. We're treated to dinosaur battles in glorious stop-motion, mixed-up gender relations and the early death of the swarthy halfbreed betrayers of the protagonists. It's all in the combination of cowboys and dinosaurs, including the spectacular mid-point roping of the Allosaurus that is alone worth the price of admission. It continues up to what is, to my knowledge, the only time in cinema history where a dinosaur masquerading as an evil spirit is captured inside of a burning cathedral church, make of the symbolism what one will.
It is also worth noting, as a point of historical interest, that The Valley of Gwangi has roots going further back than Ray Harryahusen. The concept of "Gwangi" was originally developed by Harryahusen's mentor, the great Willis O'Brien. O'Brien was the stop-motion animation pioneer behind the classics The Lost World (1925) and King Kong (1933), and looked to realize this fantastic story of cowboys and dinosaurs. Unfortunately it was never produced, and we can only imagine how the scene would have appeared in glorious black and white. Nevertheless, Harryhausen used his clout in the wake of his string of box office hits to see "Gwangi" through.