Once upon a time in Mexico, circa 1900, a Wild West Show is in financial trouble and Tuck Kirby (James Franciscus) wants to buy it from his former lover T.J. Breckenridge. There is also a Mexican boy named Lope (Curtis Arden) who wants to join the show. T.J. has a gimmick for her show—a tiny horse called El Diablo. A British paleontologist, Horace Bromley (Laurence Naismith) believes it is an Eohippus, extinct since the Eocene.

The creature came from the Forbidden Valley. A Gypsy called Tia Zorina (Freda Jackson) says it is cursed and wants it sent back. The gypsy joins with Professor Bromley to steal it away and take it back to the Forbidden Valley, but he hopes to find other specimens there. Tuck sets out after it, but Carlos (Gustave Rojo) tells T.J. that Tuck stole it. Carlos and T.J. follow Tuck.

In the valley, they find a Pteranodon, an Ormithomimus, an Epanterias they call Gwangi, and a Styracosaurus. They try to rope Gwangi, which kills Carlos, and eventually they tie it up and drag it to the Wild West Show. The Gypsy is killed setting it free. It kills Bromley and an elephant and rampages through town. It breaks into the cathedral where people have gone for sanctuary but is distracted by the organ. Tuck wounds it with a flag and accidentally sets the cathedral on fire. Gwangi is destroyed very much like the Beast from 20,000 Fathoms.

The story was conceived by Willis O’Brien of King Kong fame, Harryhausen’s mentor, inspired by Sir Arthur Conan Doyle’s The Lost World. O’Brien worked on the project for RKO, but it was cancelled on the grounds that people don’t want to see dinosaurs. O’Brien made Mighty Joe Young instead. Harryhausen inherited the project after O’Brien died. It was made by Harryhausen and producer Charles Schneer, and Warner Brothers produced it. The scene in which Gwangi is roped by cowboys on rearing horses was a difficult one, and a technical triumph in my opinion. The scene in which Gwangi pounces on the Ornithomimus has often been copied, particularly in Jurassic Park.

Gila Golan’s lines were dubbed because of her heavy Israeli accent. Laurence Naismith (Professor Bromley) was Argos in Jason and the Argonauts. Producer Schneer said the director was stupid and lost interest in the movie, so it was not what it could have been, and Warner Brothers just dumped it on the market with little publicity. It has since become a cult classic. Gwangi means lizard in a native language. The music by Jerome Moross was from his movie The Big Country, with Gregory Peck, Charlton Heston, and Chuck Connors, which gave it something of a Western scope. Cowboys with dinosaurs has to be the weirdest conflict until Cowboys and Aliens, by Steven Spielberg.