Pliohippus is a further intermediate form.
"The Story of Evolution" by Joseph McCabe
The first one-toed horse, Pliohippus, about forty inches high at the shoulder.
"The Outline of Science, Vol. 1 (of 4)" by J. Arthur Thomson
This animal lived in Europe while the Pliohippus and the Protohippus were flourishing in America.
"Earth and Sky Every Child Should Know" by Julia Ellen Rogers