bighorn sheep vs mountain sheep

bighorn sheep and mountain sheep both are nouns.

Word	Noun	Adjective	Verb	Adverb
bighorn sheep	Yes	No	No	No
mountain sheep	Yes	No	No	No

As nouns, mountain sheep is a hypernym of bighorn sheep; that is, mountain sheep is a word with a broader meaning than bighorn sheep:

bighorn sheep: wild sheep of mountainous regions of western North America having massive curled horns
mountain sheep: any wild sheep inhabiting mountainous regions

bighorn sheep (noun)	mountain sheep (noun)
wild sheep of mountainous regions of western North America having massive curled horns	any wild sheep inhabiting mountainous regions