Brontotheriidae

The Brontotheriidae, also called Titanotheriidae, is a family of extinct mammals belonging to the order Perissodactyla. This is the order that includes horses, rhinoceroses, and tapirs. On the outside, Brontotheres looked rather like rhinos, but they were actually more closely related to horses. They lived around 56–34 million years ago, until the end of the Eocene.
Later brontotheres were huge, up to 2.5 m (8.2 ft) in height with bizarre horn-like skull appendages.