pastern vs os

pastern and os both are nouns.

Word	Noun	Adjective	Verb	Adverb
pastern	Yes	No	No	No
os	Yes	No	No	No

As nouns, os is a hypernym of pastern; that is, os is a word with a broader meaning than pastern:

Other hypernyms of pastern include bone.

pastern (noun)	os (noun)
the part between the fetlock and the hoof	rigid connective tissue that makes up the skeleton of vertebrates
	a mouth or mouthlike opening