shepherd dog vs sheep dog

shepherd dog and sheep dog both are nouns.

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

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

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

Other synonyms of shepherd dog include sheepdog.

shepherd dog (noun)	sheep 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