horse of the wood vs capercailzie

horse of the wood and capercailzie both are nouns.

Word	Noun	Adjective	Verb	Adverb
horse of the wood	Yes	No	No	No
capercailzie	Yes	No	No	No

As nouns, horse of the wood and capercailzie are synonyms defined as:

Other synonyms of horse of the wood include capercaillie, Tetrao urogallus.

horse of the wood (noun)	capercailzie (noun)
large black Old World grouse	large black Old World grouse