ProtorohippusProtorohippus, + Greek: "mountain", "horse" ) is an extinct genus of equid that lived in the Eocene of North America.PalaeobiologyBased on oxygen and carbon isotope analysis of the teeth of P. montanum, the species is believed to have had two distinct birth seasons per year, a result of the diminished seasonality during the Early Eocene. It is possible that this phenomenon held for other early equids as well.image:The American Museum journal .jpg|thumb|left|Restoration by Charles R. Knight