Miohippus is reported to have a slightly longer skull than Mesohippus as well as minute ankle joint and dentine differences. He was pad-footed with three toes on all four feet with a tiny 4th vestigial (V) front toe.
Miohippus and Mesohippus (and Parahippus) are really the same animal. They are classified differently because different skeletons of the same animal can exhibit minute differences. They may be two closely related cross-fertile species like Mule deer and White Tail deer for example. The differences are also easily accounted for by if adult and juvenile specimens had been found.