Equus simplicidens, also known as the Hagerman horse and American zebra, is an extinct species of equine native to North America during the Pliocene and Early Pleistocene.

[4] Gidley believed that such a large amount of fossils found in one location was because of the quarry area being a watering hole at one point.

[4] A study by H. Gregory McDonald in 1996 alternatively suggested based on the age distribution of remains at the quarry, which span from newborns to adults, that a herd died in a single catastrophic event, perhaps attempting to cross a swelled/flooded river.

[7] Remains of Equus simplicidens are known from across Western and Central North America, including Idaho, Arizona, California, Texas, Nebraska and Kansas, southwards to Mexico.

[12] Equus simplicidens is thought to have had a highly abrasive almost exclusively grazing based diet, similar to that of modern equines, particularly zebras.