red deer vs hart

red deer and hart both are nouns.

Word	Noun	Adjective	Verb	Adverb
red deer	Yes	No	No	No
hart	Yes	No	No	No

As nouns, hart is a hyponym of red deer; that is, hart is a word with a more specific, narrower meaning than red deer:

Other hyponyms of red deer include stag, hind, brocket.

red deer (noun)	hart (noun)
common deer of temperate Europe and Asia	a male deer, especially an adult male red deer