sheepdog vs shepherd dog

sheepdog and shepherd dog both are nouns.

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

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

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

Other synonyms of sheepdog include sheep dog.

sheepdog (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