Arsinoitherium. Illustration and photo-reconstruction of the Arsinoitherium, an ancient species of Embrithopoda, walking through the African savannah. Arsinoitherium is an extinct genus of paenungulate mammal related to elephants, sirenians, hyraxes and the extinct desmostylians. They were rhinoceros-like herbivores that lived during the late Eocene and the early Oligocene of northern Africa from 36 to 30 million years ago. They lived in areas of tropical rainforest and at the margins of mangrove swamps. 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.