Breadcrumb Home > Thesaurus > P Synonyms of protohippus Find synonyms for: Noun1. protohippus, horse, Equus caballususage: Pliocene horse approaching donkeys in sizeWordNet 3.0 Copyright © 2006 by Princeton University. All rights reserved. ✖ Related Content Daily Word Quiz: sepulture Analogy of the Day: Today’s Analogy Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language