Wiktionary
posthorse
n. A horse ridden by a postrider to carry mail.
Usage examples of "posthorse".
Authorization for posthorses was therefore included, and chits for lodging in the government mansios along the way.
Towards evening all the family joined us again, a walk was proposed, and we were on the point of going out, when a carriage drawn by six posthorses noisily entered the yard.