foxhound vs hound dog

foxhound and hound dog both are nouns.

Word	Noun	Adjective	Verb	Adverb
foxhound	Yes	No	No	No
hound dog	Yes	No	No	No

As nouns, hound dog is a hypernym of foxhound; that is, hound dog is a word with a broader meaning than foxhound:

foxhound: medium-sized glossy-coated hounds developed for hunting foxes
hound dog: any of several breeds of dog used for hunting typically having large drooping ears

Other hypernyms of foxhound include hound.

foxhound (noun)	hound dog (noun)
medium-sized glossy-coated hounds developed for hunting foxes	any of several breeds of dog used for hunting typically having large drooping ears