The genus Mesohippus, of the middle Oligocene, may be selected as a kind of half-way stage in the long progression.
"Darwin and Modern Science" by A.C. Seward and Others
All the known species of this genus are larger than those of Mesohippus, and none of them pass above the Miocene formation.
"Darwinism (1889)" by Alfred Russel Wallace
Three-toed horse, Mesohippus, about the size of a sheep.
"The Outline of Science, Vol. 1 (of 4)" by J. Arthur Thomson