fetid horehound vs black horehound

fetid horehound and black horehound both are nouns.

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

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

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

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

fetid horehound (noun)	black 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