This prehistoric mammal lived in the Eocene epoch some 50 million years before the present, starting out only about a meter in length but eventually growing to a size comparable to modern asiatic elephants. The most striking feature was a blunt Y-shaped bony horn growing from the nose, reminiscent of the horns of a rhinoceros. The skull was buttressed with heavy reinforcements and supported with bulky neck muscles, suggesting the horns might have been wielded as formidable weapons. Titanotheres were odd-toed ungulates (order Perissodactyla) just as their cousins the tapirs and horses are and unlike the ruminants such as cattle. These massive creatures must have been an impressive sight, especially if they gathered into herds.
The name of the genus is Brontotherium, meaning "thunder beasts." Their fossil remains were known to the Sioux tribes of South Dakota who would find them in the rocky outcroppings in the Badlands. The family died out about 34 million years ago during the major extinction event named by Swiss palaeontologist Hans Georg Stehlin the Grande Coupure, meaning "Great Break," which wiped out many megafauna species in Eurasia as well.
Fictional Titanotheres appear in James Cameron's film Avatar as eleven meter long six-legged hammer-headed feathered creatures on the planet Pandora, but these are another matter entirely.
Sources:
http://my.execpc.com/~dschaeff/Titanotheres.htm
http://en.wikipedia.org/wiki/Brontotheriidae
http://james-camerons-avatar.wikia.com/wiki/Hammerhead_Titanothere