Here is the best part of the dino-western The Beast of Hollow Mountain (US/Mexico-1956: dir. Edward Nassour and Ismael Rodriquez): the climactic Allosaurus attack — all somewhat reminiscent of Harryhausen’s later The Valley of Gwangi (1969), what with the cowboys vs dino theme. Famed King Kong creator Willis O’Brien provided the story and was slated to handle the SFX, but in the event the job was undertaken by Edward Nassour (stop-motion animation, uncredited), Jack Rabin, Louis DeWitt and Henry Sharp. Some sources credit Marcel Delgado with making the Allosaurus model.
The Beast of Hollow Mountain was apparently the first feature film made with a widescreen aspect ratio (2.35:1), combined with stop-motion and in colour.