A large aggressive herbivore with horns for self defense
in North America durring the Late Cretaceous. In the film The Valley of Gwangi, some North American Late Cretaceousdinosaurs, including Styracosaurus, are living a remote North American valley in Mexico. Although there isn't a large supply of vegatation to sustane large Styracosaurus, the film is still realistic do to the fact that all of the dinosaurs are North American and from the Late Cretaceous, the last era of the Mesozoic, compared to One Million Years B.C. where dinosaurs, and people, from different times are mixed together.
It is also possible that the styracosauri in the valley, if more were found were in fact carnivore which would give an explaintion to the creature's continious aggressive behaviour.