sheep dog vs working dog

sheep dog and working dog both are nouns.

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

As nouns, working dog is a hypernym of sheep dog; that is, working dog is a word with a broader meaning than sheep dog:

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

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