Stygimoloch belongs to to the family Pachycephalosauridae, ornithischian dinosaurs that had thick, domed, spiked skulls. Pachycephalosaurs probably engaged in head-butting both as a defense and in intra-species rivalry.

Anatomy: Stygimoloch's thick skull had many bony spikes and knobs; the horns ranged up to 4 inches (10 cm) long. Stygimoloch was about 7 feet (2 m) long, weighing roughly 170 pounds (78 kg).

Name and Fossils: Stygimoloch was named for a demon from the river Styx (a mythical river that one must cross to enter Hades [Hell]). Only parts of skulls of Stygimoloch have been found (in the Hell Creek formation, Montana and Wyoming, USA). Stygimoloch was named in 1983 by British paleontologist Peter M. Galton and German paleontologist Hans-Dieter Sues. The type species is S. spinifer.