In this report we describe a large sample of Onohippidium galushai, new species, from the Late Hemphillian (Early Pliocene) of western Arizona, one ramus of Hippidion sp. from the Early Hemphillian (Late Miocene) of the Texas High Plains, and one ramus of cf. Hippidion sp. from the Irvingtonian (Late Pliocene to Early Pleistocene) of southern California. These one-toed horses were previously thought to have been confined to Plio-Pleistocene deposits of South America. Onohippidium from Arizona possesses a retracted nasal notch, deep preorbital facial fossae, and dental pattern diagnostic of South American representatives of this genus. Hippidion from Texas and California exhibit the dental characters, especially in the deep ectoflexids, seen in South American representatives of this genus. This is the first report of these horses from North America. The presence of Onohippidium and Hippidion in North America demonstrates that diversification of these "endemics" occurred prior to dispersal to South America during the Pleistocene great faunal interchange.