Stygimoloch is a disputed genus of pachycephalosaur from the end of the Cretaceous period, roughly 66 million years ago. Its fossils have been found across Western North America but primarily hail from the Hell Creek Formation in Montana. Though the first known specimen was discovered in the 1800s, it was first recognised as a distinct genus in 1983 based on horned squamosal bones and given the name Stygimoloch spinifer. Its name is derived from the mythological river Styx, Moloch, and the Latin term for possessing spines. An isolated dome was given the distinct genus and species Stenotholus kohleri in 1987, but shortly thereafter the discovery of a more complete skull including a dome and squamosals indicated it was a junior synonym of Stygimoloch. It can be distinguished from other pachycephalosaurs by its possession clusters of elongate spikes on the back of its head and the especially narrow form of its dome. Classically interpreted as a close relative of the contemporary genus Pachycephalosaurus, a 2009 theory posited that it alongside Dracorex is instead a juvenile form of Pachycephalosaurus, rather than a distinct animal. Its taxonomic status remains in dispute, with some studies treating it as a synonym of Pachycephalosaurus while others retain it as a distinct genus that may have lived after Pachycephalosaurus.