Equus (genus) 54 million years ago (Mya). These animals had three toes on the hind feet and four on the front feet with small hooves in place of claws, but also had soft pads.<ref name="doi10.1126/science.1105458"></ref> Equids developed into larger, three-toed animals (e.g. "Mesohippus") during the Oligocene and Miocene. From there, the side toes became progressively smaller through the Pleistocene until the emergence of the single-toed "Equus". The genus "Equus", which includes all extant equines, is believed to have evolved from "Dinohippus", via the intermediate form "Plesippus". One of the oldest species is "Equus simplicidens", described as zebra-like with a donkey-like head shape.
Equus conversidens New Mexico, Oklahoma, Kansas and Florida, synonymizing "Equus francisci", "Equus tau", "E. littoralis", "E. achates", and "E. barcenaei" with "E. conversidens". However, Winans (1985) and MacFadden (1992) challenged the validity of "E. conversidens" due to its minimal diagnostic value, and treated "E. francisci" as valid. Subsequent study confirmed the "nomen dubium" status of "E. conversidens" because the holotype was undiagnostic besides its small size. Equus conversidens Equus conversidens (Owen 1869), or the Mexican horse, is a dubious Pleistocene species of horse, now extinct, that inhabited North America. The holotype of "Equus conversidens", a partial palate, was unearthed in Pleistocene deposits