A large purplish Allosaur (a species of extinct carnivorous theropod dinosaur), Gwangi did not resemble an Allosaurus as we believe they appeared today, rather he was based on the suppositions of the time, being portrayed as considerably more "upright" than current scientific theory holds.
At the turn of the century, after discovering that a tiny horse a local rodeo owner has in her care is actually a prehistoric Eohippus, a group of superstitious gypsies who believe that it should be returned to the Forbidden Valley where it was found kidnap the beast and head to the canyon. Followed by a posse of cowboys from the rodeo, both parties enter the valley where they discover the fearsome Allosaur Gwangi, who attempts to kill and eat the group. After an effort to lasso the beast fails, the cowboys retreat through a narrow crevasse, where a rockfall knocks Gwangi unconscious. The rodeo party then bind the creature and transport it back to town where they intend to put it on show.On the opening day of the show, one of the gypsies, still believing that anything from the Forbidden Valley is cursed and should not be permitted among men, tries to free Gwangi but the dinosaur breaks free and embarks upon a rampage through the town, battling and killing one of the circus elephants in the process. Many of the townsfolk seek sanctuary in the local cathedral, but Gwangi smashes its way into the building in search of prey. Ushering the townspeople out of the back door, one of the cowboys succeeds in setting the church ablaze, burning the poor creature to death and ending its threat.