Brontotherium ('thunder beast') is an extinct genus of prehistoric perissodactyl mammal of the family Brontotheriidae, an extinct group of rhinoceros-like browsers related to horses. The genus was found in North America during the Late Eocene. The creature stood 2.50 m (8 ft 4 in) tall at the shoulders and resembled a rhinoceros, possessing a forked (in a Y, or slingshot shape), horn-like protrusion on its nose, with blunt ends. Many Brontotherium remains have been found in South Dakota and Nebraska. In the past, specimens exposed by severe rainstorms were found by Native Americans of the Sioux tribe. The Sioux believed these creatures produced thunderstorms when running over the clouds, and called them 'thunder horses'. Many of the skeletons found by the Sioux belonged to herds which were killed by volcanic eruptions of the Rocky Mountains, which were volcanically active at the time.

Brontotherium's dorsal vertebrae above the shoulders had extra long spines to support the huge neck muscles needed to carry the heavy skull. Possibly, Brontotherium had fleshy lips and a long tongue, perfect for carefully selecting preferred food (soft stems and leaves).

According to Mihlbachler et al. 2004, the species within genus Brontotherium were merged with the genus Megacerops, though not all authorities agree on this.