Pachycephalosaurus wyomingensis ought to be a familiar name to most of you. Discovered in Wyoming in 1931, it has long been famous as the dinosaur with the domed bony head, supposedly used by males in territorial and breeding disputes much like bighorn sheep today, by charging and then headbutting each other in a test of endurance and strength. Recently that thought has been disputed, as the skull likely could not have sustained such blows, nor could the neck, and no evidence of scarring or other damage on the heads of pachycephalosaurs that would result from this kind of combat has been found. They may instead have headbutted the side or flank of competing males, or of potential predators in self-defense.
Juveniles had far more extensive head spines than did adults, which initially caused paleontologists to think they were different species - Stygimoloch spinifer and Dracorex hogwartsia (what a name!). It now seems that the horns on young Pachycephalosaurus were absorbed into the heavy head dome as bone grew around them during the aging process.
Pachycephalosaurus were beaked but had small leaf-shaped teeth - unlike their relatives the ceratopsians, they must have eaten leaves, nuts, soft fruits, and/or insects, rather than tough cycads and grasses. The end of the Cretaceous had plenty of soft leafy plants and flowers for them to eat, making them less similar to cows and more like today's deer in terms of diet.