Stygimoloch is a genus of pachycephalosaurid dinosaur that lived during the Late Cretaceous period, around 66 million years ago. The name Stygimoloch means “demon from the river Styx,” referencing its fierce appearance and the river that, according to Greek mythology, separated the world of the living from the world of the dead.

Stygimoloch was a small dinosaur, reaching around 10 feet in length and weighing up to 300 pounds. It had a thick, bony skull, which it likely used for head-butting during intraspecific combat. Its skull was adorned with multiple spikes and bumps, making it one of the most unusual-looking dinosaurs.

Despite its fearsome appearance, it was likely a herbivore, feeding on tough plant material. Its fossils have been found in North America, in the states of Montana, Wyoming, and South Dakota. It is often considered a junior synonym of the genus Pachycephalosaurus.