Pachycephalosaurus

Its name means thick-headed reptile. It had an incredibly thick skull (25cm), a tiny brain and large eyes, suggesting it had good vision. By lowering its head, its body became completely horizontal giving it the ability to ram like a battering ram. It was a grazing dinosaur that lived in small groups. It is believed that it was the adult growth stage of the same species, along with Dracorex and Stygimoloch.