whitetail deer vs deer

whitetail deer and deer both are nouns.

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

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

whitetail deer: common North American deer; tail has a white underside
deer: distinguished from Bovidae by the male's having solid deciduous antlers

Other hypernyms of whitetail deer include cervid.

whitetail deer (noun)	deer (noun)
common North American deer; tail has a white underside	distinguished from Bovidae by the male's having solid deciduous antlers