sheep dog vs shepherd dog

sheep dog and shepherd dog both are nouns.

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

As nouns, sheep dog and shepherd dog are synonyms defined as:

sheep dog and shepherd dog: any of various usually long-haired breeds of dog reared to herd and guard sheep

Other synonyms of sheep dog include sheepdog.

sheep dog (noun)	shepherd dog (noun)
any of various usually long-haired breeds of dog reared to herd and guard sheep	any of various usually long-haired breeds of dog reared to herd and guard sheep