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