Megacerops coloradensis. Computer illustration and photo-reconstruction of the prehistoric rhino Brontotherium Megacerops coloradensis. Megacerops is an extinct genus of the prehistoric odd-toed ungulate family Brontotheriidae, an extinct group of rhinoceros-like browsers related to horses. Brontotheriidae is a family mammal that lived in Palaeogene (from lower Eocene until lower Oligocene) in North America, Asia and Europe. They stood about 2.5 meters tall at the shoulders and fed on relatively soft vegetation. Scientists consider climate change and consequent change in vegetation the main reason for the extinction of these rhinoceros-like thunder horses. Another possible reason for their brief existence was the small size of their brains and imperfect teeth.