harness horse vs cob

harness horse and cob both are nouns.

Word	Noun	Adjective	Verb	Adverb
harness horse	Yes	No	No	No
cob	Yes	No	No	No

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

Other hyponyms of harness horse include hackney.

harness horse (noun)	cob (noun)
horse used for pulling vehicles	adult male swan
	stocky short-legged harness horse
	white gull having a black back and wings
	nut of any of several trees of the genus Corylus