What is "brontotheriidae"

Brontotheriidae, also called Titanotheriidae, is a family of extinct mammals belonging to the order Perissodactyla, the order that includes horses, rhinoceroses, and tapirs. Superficially they looked rather like rhinos, although they were actually more closely related to horses. They lived around 56–34 million years ago, until the very close of the Eocene.