I mean, no prehistoric creature is ever going to be dungeon-only critter- all of them are creatures that had an ecological niche in the real world once upon a time. But maybe, instead of just giving us huge mammals, talk about a 'forest' that has colossal mushrooms instead of trees. Populate the seas with bizarre Cambrian monsters like Anomalocaris, Opabinia and Hallucigenia. Populate the land with ridiculous Triassic reptiles like Tanystropheus and Sharovipteryx. There's the potential for really weird and interesting creatures, even without any departures from the real life critters.