The most bizarre animal that shared the Mongolian landscape in the Oligocene with the before mentioned animals must have been the Chalicothere. They were large animals that walked with most of their weight on their short but strong hind legs but also bearing weight on their knuckles of their extremely long forelimbs. They had huge long claws on their front limbs, which necessitated the knuckle walking. They used their claws to hook down branches to get to young soft leaves. They seemed to have been fussy eaters (fossil teeth show little wear), settling for only young shoots or soft leaves. It is hard to imagine that these animals were as tall as 2,6 meters when resting and could weigh up to 350 kilograms. They are distantly related to horses, rhinos and tapirs. Again the fossil record comes from Asia. (21)
Why did these mammals of the Oligocene go extinct? Probably again due to changes in the climate that became colder and drier, eventually causing desserts to form and changing rainforests into deciduous woodlands and woodlands into savannah.