Stygimoloch was a genus of the pachycephalosaurid family of dinosaurs, which had thick bone structures on their heads. The Stygimoloch lived in North America during the Late Cretaceous, and was found in the states of Montana, South Dakota and Wyoming. One of the main differences it had from other Pachycephalosaur genus was the row of spikes along the back of the head.
In 2009, Jack Horner and M.B. Goodwin published a paper with documented evidence to support their belief that Stygimoloch, along with the Dracorex, were not individual genus but in fact merely different stages of a Pachycephalosaurus‘ youth. There paper noted that the spike, or node, as well as the skull domes of all three species had extreme plasticity, meaning they were very easily shaped or molded. Another point of evidence they made was that both Dracorex and Stygimoloch have only been found in a juvenile form, while Pachycephalosaurus has only been found in adult form. They also made note of the fact that all three lived in the same locations. This, along with further research done by Nick Longrich in 2010, has led the paleontology community for the most part to believe that Stygimoloch and Dracorex were simply juvenile forms of Pachycephalosaurus, and not their own individual genus.
The dinosaur appears in Jurassic World: Fallen Kingdom, much to the surprise of many fans as we had not known about its existence on Isla Nublar beforehand. However, we are only introduced to it after we leave the island, and get to Lockwood’s Manor. While Claire and Owen are locked up in one of the holding cells, they hear the grunting of the dinosaur in the cell next to them. Nicknamed “Stiggy”, they use the dinosaur to help them escape by agitating it which eventually leads to it bursting through the wall and then through their cell door. We later see the animal running amok in the auction room. Five of the animals were supposedly rescued from Isla Nublar, and when Claire and Maisie let them free we see some of them escaping alongside the other dinosaur species.
Mattel has released figures of the Stygimoloch for their various Jurassic World toylines, and the dinosaur appeared in Jurassic World Evolution as part of the Jurassic World: Fallen Kingdom dinosaur pack.