Temporal range: Ypresian,
|Reconstructed skeleton, National Museum of Natural History, Washington, DC, United States|
Eohippus is an extinct genus of small equid ungulates. The only species is E. angustidens, which was long considered a species of Hyracotherium. Its remains have been identified in North America and date to the Early Eocene (Ypresian stage).
In 1876, Othniel C. Marsh described a skeleton as Eohippus validus, from Greek: ἠώς (eōs, 'dawn') and ἵππος (hippos, 'horse'), meaning 'dawn horse'. Its similarities with fossils described by Richard Owen were formally pointed out in a 1932 paper by Clive Forster Cooper. E. validus was moved to the genus Hyracotherium, which had priority as the name for the genus, with Eohippus becoming a junior synonym of that genus. Hyracotherium was recently found to be a paraphyletic group of species, and the genus now includes only H. leporinum. E. validus was found to be identical to an earlier-named species, Orohippus angustidens Cope, 1875, and the resulting binomial is thus Eohippus angustidens.
Eohippus stood at about 12 in (30 cm), or 3 hands tall. It had 4 toes on its front feet and 3 toes on the hinds, each toe ending in a hoof. Its incisors, molars and premolars resemble modern Equus, however a differentiating trait of Eohippus is its large canine teeth. 
Stephen Jay Gould comments
In his 1991 essay, "The Case of the Creeping Fox Terrier Clone", Stephen Jay Gould lamented the prevalence of a much-repeated phrase to indicate Eohippus size ("the size of a small Fox Terrier"), even though most readers would be quite unfamiliar with that breed of dog. He concluded that the phrase had its origin in a widely-distributed pamphlet by Henry Fairfield Osborn, and proposed that Osborn, a keen fox hunter, could have made a natural association between his horses and the dogs that accompanied them. Eohippus was approximately 12 in (30 cm) high at the shoulder, which would be comparable to a Fox Terrier, which is up to 13 in (33 cm) high at the shoulder.
- MacFadden, B. J. (18 March 2005). "Fossil Horses--Evidence for Evolution". Science. 307 (5716): 1728–1730. doi:10.1126/science.1105458. PMID 15774746.
- Froehlich, D. J. (2002). "Quo vadis eohippus? The systematics and taxonomy of the early Eocene equids (Perissodactyla)". Zoological Journal of the Linnean Society. 134 (2): 141–256. doi:10.1046/j.1096-3642.2002.00005.x.
- Cope, E.D. (1875). Systematic Catalogue of Vertebrata of the Eocene of New Mexico, collected in 1874. p. 22.
- "Hyracotherium (Eohippus)". University of Guelph. Archived from the original on 2020-11-13.
- "Eohippus | Size & Facts | Britannica". www.britannica.com. Retrieved 2022-09-27.
- Gould, S. J. (1991). "Essay 10: The case of the creeping fox terrier clone". Bully for Brontosaurus: Reflections in Natural History. W.W. Norton & Co. ISBN 978-0-393-02961-1.
- "Fox Terrier (Vital Stats)". DogTime. Archived from the original on 2021-01-26.