workhorse vs farm horse

workhorse and farm horse both are nouns.

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

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

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

workhorse (noun)	farm horse (noun)
a horse used for plowing and hauling and other heavy labor	a quiet plodding workhorse
machine that performs dependably under heavy use