black horehound vs fetid horehound

black horehound and fetid horehound both are nouns.

Word	Noun	Adjective	Verb	Adverb
black horehound	Yes	No	No	No
fetid horehound	Yes	No	No	No

As nouns, black horehound and fetid horehound are synonyms defined as:

black horehound and fetid horehound: ill-smelling European herb with rugose leaves and whorls of dark purple flowers

Other synonyms of black horehound include Ballota nigra, black archangel, stinking horehound.

black horehound (noun)	fetid horehound (noun)
ill-smelling European herb with rugose leaves and whorls of dark purple flowers	ill-smelling European herb with rugose leaves and whorls of dark purple flowers