Simplify the following text for a Grade 10 native English-speaking person.
Original: Arsinoitherium is an extinct genus of paenungulate mammal related to elephants , sirenians , hyraxes and the extinct desmostylians , as well as to other extinct embrithopods . These species were rhinoceros-like herbivores that lived during the late Eocene and the early Oligocene of northern Africa from 36 to 30 million years ago , in areas of tropical rainforest and at the margin of mangrove swamps ; a newly discovered species , Arsinoitherium giganteum , lived in Ethiopia ~ 27 million years ago .
Simplified: Arsinoitherium is a extinct genus , a large mammal that lived during the early Oligocene ( about 38 to 23 million years ago ) .