It had three toes on each foot and is the first horse known to have grazed. Its name means "ruminant horse", but current evidence does not support Merychippus ruminating.
Merychippus was named by Leidy (1856). Numerous authors around the turn of the century put the type species in Protohippus, but this ignored priority. Its type is Merychippus insignis. It was assigned to Equidae by Leidy (1856) and Carroll (1988); and to Equinae by MacFadden (1998) and Bravo-Cuevas and Ferrusquía-Villafranca (2006).
Four specimens were examined by M. Mendoza, C. M. Janis, and P. Palmqvist for body mass. The results were:
- Specimen 1: 164.7 kg (360 lb)
- Specimen 2: 106.4 kg (230 lb)
- Specimen 3: 58.9 kg (130 lb)
- Specimen 4: 17.2 kg (38 lb)