For 10 points, name this dinosaur that had three horns on its skull.
Upon discovering the remains of these creatures, Othniel Marsh believed the bones belonged to an extinct bison.  Scannella and Horner proposed in July that the variation in shape between skulls of this animal suggests that it is an earlier growth stage of the previously distinct genus Torosaurus. These animals had a large, epoccipital-bone-lined frill at the back of their skulls. This herbivorous dinosaur shared the Late Cretaceous Period with its main predator, the Tyrannosaurus Rex. For 10 points, name this dinosaur that had three horns on its skull.
Upon discovering the remains of these creatures, Othniel Marsh believed the bones belonged to an extinct bison.  
triceratops