Elasmotherium

The largest of all the prehistoric rhinoceroses of the Pleistocene epoch, Elasmotherium was a truly massive piece of megafauna, and all the more imposing thanks to its thick, shaggy coat of fur (this mammal was closely related to Coelodonta, also known as the "woolly rhino") and the huge horn on the end of its snout. This horn, which was made of keratin (the same protein as human hair), may have reached five or six feet in length--and if Elasmotherium survived into historical times, it's possible that early humans glimpsing this huge, strange beast may have been inspired to create the legend of the unicorn.

For all its size, bulk and presumed aggressiveness, though, Elasmotherium was still a relatively gentle herbivore--and one well-adapted to eating grass rather than leaves or shrubs, as evidenced by its almost comically heavy, oversized, flat teeth and lack of incisors.