Uintatherium robustum mammal, illustration. This mammal lived during the Eocene, some 52 to 36 million years ago. It belonged to the same group as horses, rhinos and tapirs, although it most resembles modern-day rhinoceroses, or elephants. They were herbivores and evolved to huge body sizes, up to 1.6 metres high at the shoulder, and 4 metres long. Their heads were adorned with six bony protrusions and their upper jaws grew a pair large dagger-like tusks. This artwork depicts a specimen from North America that dates to 50 million years ago.