Protohippus is an extinct three-toed genus of horse. It was roughly the size of a modern donkey. Fossil evidence suggests that it lived during the Late Miocene (Clarendonian to Hemphillian), from about 13.6 Ma to 5.3 Ma.
|Protohippus simus skeleton|
|Equus (Protohippus) perditus|
- P. vetus
- P. perditus
- P. supremus (also P. simus)
- P. gidleyi
- Leidy, Joseph (1858). "Notice of Remains of Extinct Vertebrata, from the Valley of the Niobrara River, collected during the Exploring Expedition of 1857, in Nebraska, under the command of Lieut. G. K. Warren, U. S. Top. Eng., by Dr. F. V. Hayden, Geologist to the Expedition". Proceedings of the Academy of Natural Sciences of Philadelphia. 10: 20–29. JSTOR 4059296.
- Leidy, Joseph (1869). "The Extinct Mammalian Fauna of Dakota and Nebraska: Including an Account of Some Allied Forms from Other Localities, Together with a Synopsis of the Mammalian Remains of North America". Journal of the Academy of Natural Sciences of Philadelphia. Ser. 2. 7: 275–279, 401.
- MacFadden, Bruce J.; Dobie, James L. (1998). "Late Miocene Three-Toed Horse Protohippus (Mammalia, Equidae) from Southern Alabama". Journal of Paleontology. 72 (1): 149–152. JSTOR 1306685.
- Hulbert, Richard C., Jr. (1988). "Calippus and Protohippus (Mammalia, Perissodactyla, Equidae) from the Miocene (Barstovian-Early Hemphillian) of the Gulf Coastal Plain" (PDF). Bulletin of the Florida State Museum Biological Sciences. 32 (3): 221–340.