High Quality Content by WIKIPEDIA articles! The Hagerman horse (Equus simplicidens), also called the Hagerman zebra or the American zebra, was a North American species of equid from the Pliocene period and the Pleistocene period. It was one of the oldest horses of the genus Equus. Discovered in 1928 in Hagerman, Idaho, it is believed to have been like the Grevy`s zebra of East Africa. It is the state fossil of Idaho.