Miohippus lived during the Oligocene Period some 25 to 40 million years ago. It branched off from Mesohippus, and the two coexisted for about four million years. Although Mesohippus had died out by the mid-Oligocene, Miohippus grew rapidly.

Miohippus was larger than Mesohippus and had a slightly longer skull. Its facial fossa became deeper and more expanded, and the ankle joint changed subtly. Miohippus also had a variable extra crest on its upper cheek teeth, which is now a typical characteristic of the teeth of later equine species.