mule deer vs deer

mule deer and deer both are nouns.

Word	Noun	Adjective	Verb	Adverb
mule deer	Yes	No	No	No
deer	Yes	No	No	No

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

mule deer: long-eared deer of western North America with two-pronged antlers
deer: distinguished from Bovidae by the male's having solid deciduous antlers

Other hypernyms of mule deer include cervid.

mule deer (noun)	deer (noun)
long-eared deer of western North America with two-pronged antlers	distinguished from Bovidae by the male's having solid deciduous antlers