Illustration of Tyrannosaurus rexes fighting. Often called T. rex or colloquially T-Rex, they were one of the most well-represented of the large theropods. They were carnivores and primarily ate herbivorous dinosaurs, including Triceratops. Tyrannosaurus rexes may not have been lone operators and may have hunted in packs. They lived 68 million years ago during the Cretaceous Period throughout what is now western North America.