An artistic 3D rendering of Brontotherium, an extinct prehistoric North American mammal which existed during the late Eocene epoch, roughly 38 million years ago. Brontotherium (also known as Megacerops) resembled a large rhinoceros, measuring about 8 feet high and 16 feet long, with a distinct Y-shaped horn which was more prominant in males. Despite its appearance, Brontotherium was more closely related to prehistoric horses as a perissodactyl (odd-toed ungulate).