The Skeleton Rex are the reanimated remains of a tyrannosaur that still roam the Skeleton Cave. It is a bulky and resilient enemy that paces and sprints in a most peculiar manner. Its attacks consist in biting with its powerful jaws, building up a pounce that lands with a snap, and breathing an elemental stream.
In the game's French and Italian versions, the enemy description says it's a brontosaurus' skeleton, despite it looking more like a tyrannosaur. This is probably a confusion due to the brontosaurus's name etymology, coming from the ancient Greek βροντή (thunder) and σαῦρος (lizard), and whose English description says it's a thunder lizard skeleton.