work of art vs warhorse

work of art and warhorse both are nouns.

Word	Noun	Adjective	Verb	Adverb
work of art	Yes	No	No	No
warhorse	Yes	No	No	No

As nouns, warhorse is a hyponym of work of art; that is, warhorse is a word with a more specific, narrower meaning than work of art:

work of art: art that is a product of one of the fine arts (especially a painting or sculpture of artistic merit)
warhorse: a work of art (composition or drama) that is part of the standard repertory but has become hackneyed from much repetition

Other hyponyms of work of art include magnum opus, art object, objet d'art, piece, pastiche, period piece.

work of art (noun)	warhorse (noun)
art that is a product of one of the fine arts (especially a painting or sculpture of artistic merit)	horse used in war
	a work of art (composition or drama) that is part of the standard repertory but has become hackneyed from much repetition
	an experienced person who has been through many battles; someone who has given long service