This is in contrast to the most common equid of the period, "Miohippus". Mating occurs in the small intestine of the equid. Eurohippus is an extinct genus of equid ungulate. Eohippus is an extinct genus of small equid ungulates. The Grevy's zebra is the unique wild equid of the region. Equus sivalensis is an extinct equid, discovered in the Siwalik hills. The fossils of the prehistoric equid, "Hipparion" were also found on the island. A 2004 study found it to be an equid instead. There are currently six species in this genus including the type species "Equid gammaherpesvirus 2". Protorohippus is an extinct equid that lived in the Eocene of North America. This quarry has produced the remains of six other equid genera including "Dinohippus" and "Nannippus". Sinohippus is an extinct equid genus belonging to the subfamily Anchitheriinae. Equid hooves are the result of the 55-million-year evolution of the horse. "Sifrhippus" is the oldest known equid, living during the early Eocene. Sifrhippus is an extinct genus of equid containing the species "S. sandrae" and "S. grangeri".