Type: Hyracodon nebraskensis ("Running Rhino & Hyrax Tooth") tooth fossil.
Size: 3.5cm long
Location: Custer Country, South Dakota, USA. 30-35 MYO
It was a lightly built, pony-like mammal of about 1.5 m (5 ft) long. Hyracodon's dentition resembled that of later rhinoceroses, but it was a much smaller animal and differed very little in appearance from the primitive horses of which it was a contemporary. Like the primitive horses, hyracodonts inhabited open forests and wooded steppes and turned from browsing foliage to grazing grass. They died out without leaving any descendants and they mark the end of the phylogenetic branch of hornless, running rhinoceroses.