oldest known genus of the horse family, about the size of a fox and first known from fossil remains found in New Mexico, 1879, Modern Latin, from eo- "earliest" + Greek hippos "horse" (from PIE root *ekwo- "horse").
word-forming element, used from mid-19c. (first in Eocene) in compound words formed by earth-scientists, and meaning "characterized by the earliest appearance of," from Greek ēōs "dawn, morning, daybreak," also the name of the goddess of the morning, from PIE root *aus- (1) "to shine," especially of the dawn. Piltdown Man, before exposed as a fraud, was known as Eoanthropus.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit açva-, Avestan aspa-, Greek hippos, Latin equus, Old Irish ech, Old Church Slavonic ehu-, Old English eoh, Gothic aihwa- all meaning "horse."
<a href="https://www.etymonline.com/word/Eohippus">Etymology of Eohippus by etymonline</a>
Harper, D. (n.d.). Etymology of Eohippus. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/Eohippus