Arsinoitherium

Arsinoitherium was a large prehistoric mammal from the late Eocene period. It is famous for its two horns on its head.

Description
When alive, it would have superficially resembled a rhinoceros, and have been about 1.8 metres tall at the shoulders, 3 metres long with a weight over 2.5 tonnes. The most noticeable feature of Arsinoitherium was a pair of enormous knife-like horns with cores of solid bone that projected from above the nose, and a second pair of tiny, knob-like horns on top of the head, immediately behind the larger horns. The skeleton is robust but shows that it was descended from a cursorial ancestor, and that the beast may have been able to run if it had to, like a modern elephant or rhinoceros. Its limb bones also suggest that the columnar legs of the living animal were elephant-like (especially since they ended in five-toed feet), rather than rhinoceros-like. Arsinoitherium had a full complement of 44 teeth, which is the primitive state of placental mammalian dentition, suggesting that it was a selective browser. The large size and hefty build of Arsinoitherium would have rendered it largely immune to predation. However, creodonts may have preyed on the young or infirm.

Horn function
In the book, The Macmillan Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals, the authors erroneously claim that the larger pair of horns of Arsinoitherium were hollow and cone-like, a claim which was repeated in the later edition, ''The Simon & Schuster Encyclopedia of Dinosaurs & Prehistoric Creatures. A Visual Who's Who of Prehistoric Life would be used by Tim Haines and Paul Chambers bolster their claim in their book, The Complete Guide to Prehistoric Life, that Arsinoitherium used its hollow horns as a sound resonator, much in the same manner the crests of lambeosaurine dinosaurs, such as Parasaurolophus'', are believed to have been used.

The Fourth Most Deadliest Sea
A pile of Arsinoitherium dung was found by Nigel Marven. He then followed a set of footprints made by an Arsinoitherium until he found it browsing. Nigel offered it an apple from the 21st century but it charged at Nigel, mistaking him as a threat. Nigel managed to escape whilst it jumped into the mangrove water to swim. Nigel followed the Arsinoitherium before then followed a pair of Dorudon.