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 niobrarensis Equus niobrarensis Equus niobrarensis (or commonly, Niobrara horse) is an extinct species of "Equus", the genus that includes the horse. "E. niobrarensis" may be synonymous with "Equus scotti". It was "stout-legged" and belonged to the "big horses" category as defined by M. C. Winans. The skull of the horse was noted as being broader than Equus caballus. "E. niobrarensis" was native to North America and commonly found in western North America and dates to the late Pleistocene. Fossils of "E. niobrarensis" were discovered at Dry Cave in Wisconsin. Another mention of an "E. niobrarensis" discovery was in Skeleton Cave, Oregon.