workhorse vs plow horse

workhorse and plow horse both are nouns.

Word	Noun	Adjective	Verb	Adverb
workhorse	Yes	No	No	No
plow horse	Yes	No	No	No

As nouns, plow horse is a hyponym of workhorse; that is, plow horse is a word with a more specific, narrower meaning than workhorse:

Other hyponyms of workhorse include plough horse, draft horse, draught horse, dray horse, packhorse, dobbin, farm horse.

workhorse (noun)	plow horse (noun)
a horse used for plowing and hauling and other heavy labor	a horse used to pull a plow
machine that performs dependably under heavy use