working dog vs sheepdog

working dog and sheepdog both are nouns.

Word	Noun	Adjective	Verb	Adverb
working dog	Yes	No	No	No
sheepdog	Yes	No	No	No

As nouns, sheepdog is a hyponym of working dog; that is, sheepdog is a word with a more specific, narrower meaning than working dog:

working dog: any of several breeds of usually large powerful dogs bred to work as draft animals and guard and guide dogs
sheepdog: any of various usually long-haired breeds of dog reared to herd and guard sheep

working dog (noun)	sheepdog (noun)
any of several breeds of usually large powerful dogs bred to work as draft animals and guard and guide dogs	any of various usually long-haired breeds of dog reared to herd and guard sheep