The third family
of rhinoceroses is that of the Rhinoceritidae or the "true rhinoceroses". It is
more certain than that of the Amynodonts, that the Rhinocerotidae were derived
from the more primitive Hyracodonts. True rhinoceroses first became prominent in the Oligocene.
These forms tended to large size and stout limbs similar to the Amynodonts. Primitive
Oligocene rhinoceroses were deemed hornless. The reason for this is not quite understood.
Elasmotherium, a strange looking beast that was the size of a modern day elephant, had a horn that grew up 2m (6.5 ft.) in length.