Mesohippus is an extinct genus of early horse. It lived some 40 to 30 million years ago from the late Eocene

Eocene

The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...

 to the mid-Oligocene

Oligocene

The Oligocene is a geologic epoch of the Paleogene Period and extends from about 34 million to 23 million years before the present . As with other older geologic periods, the rock beds that define the period are well identified but the exact dates of the start and end of the period are slightly...

. Like many fossil horses, Mesohippus was common in North America.
Mesohippus had longer legs than its predecessor Hyracotherium

Hyracotherium

Hyracotherium , also known as Eohippus or the dawn horse, is an extinct genus of very small perissodactyl ungulates that lived in the woodlands of the northern hemisphere, with species ranging throughout Asia, Europe, and North America during the early Tertiary Period and the early to mid Eocene...

and stood about 60 cm {6 hands} tall. It had also lost a toe and stood predominantly on its middle toe, although the other two were also used.