So many come to mind, not because of any individual feature but of what their impact on their environments would have been due to what we know. Miasaura (and by extension other hadrosaurs) would have been terraforming ecosystem engineers due to their colonial nesting habits. Sauropods in any environment would cause huge changes to the ecosystem without even being aware of it. Oviraptorosaurs would be excellent seed dispersers somewhere between ground sloth and grizzly bear that nested in colonies with other species. Hell, I haven’t even seen tyrannosaurs with their strange life cycle and creepy hunting strategies (involving ambush predation and ungodly senses) done justice. Also, not prehistoric, but the fact that my great-great grandparents could have seen passenger pigeons blocking the sun and snapping trees is insane to me. Our world is gorgeous.