post-horse vs Equus caballus

post-horse and Equus caballus both are nouns.

Word	Noun	Adjective	Verb	Adverb
post-horse	Yes	No	No	No
Equus caballus	Yes	No	No	No

As nouns, Equus caballus is a hypernym of post-horse; that is, Equus caballus is a word with a broader meaning than post-horse:

post-horse: a horse kept at an inn or post house for use by mail carriers or for rent to travelers
Equus caballus: solid-hoofed herbivorous quadruped domesticated since prehistoric times

Other hypernyms of post-horse include horse.

post-horse (noun)	Equus caballus (noun)
a horse kept at an inn or post house for use by mail carriers or for rent to travelers	solid-hoofed herbivorous quadruped domesticated since prehistoric times