Paraceratherium is an extinct genus of hornless rhinoceros. It may be horned or naturally hornless. Rams have horns and ewes are polled (hornless). In comparative mythology as well, horned dragons are generally more common than hornless ones. Rams and ewes are polled (hornless). Aphelops is an extinct genus of hornless rhinoceros endemic to North America. Only 20 percent of the ewes are horned; the rest are polled (hornless). Polled Holsteins have the dominant polled gene which makes them naturally hornless. It may be horned or hornless, and tassels may be present. Sexually dimorphic, females are hornless and smaller than males. This small hornless ancestor resembled a tapir or small horse more than a rhino. Many breeds of cattle and sheep are naturally hornless. Dairy breeds of goats required to be naturally hornless or disbudded. Early animals were the Entelodon, Paraceratherium (a hornless rhinoceros relative) and Basilosaurus (an early whale). The Bohuskulla is an endangered Swedish breed of hornless mountain cattle.