black-tailed deer vs mule deer

black-tailed deer and mule deer both are nouns.

Word	Noun	Adjective	Verb	Adverb
black-tailed deer	Yes	No	No	No
mule deer	Yes	No	No	No

As nouns, mule deer is a hypernym of black-tailed deer; that is, mule deer is a word with a broader meaning than black-tailed deer:

black-tailed deer: mule deer of western Rocky Mountains
mule deer: long-eared deer of western North America with two-pronged antlers

Other hypernyms of black-tailed deer include burro deer, Odocoileus hemionus.

black-tailed deer (noun)	mule deer (noun)
mule deer of western Rocky Mountains	long-eared deer of western North America with two-pronged antlers