sheep dog vs collie

sheep dog and collie both are nouns.

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

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

sheep dog: any of various usually long-haired breeds of dog reared to herd and guard sheep
collie: a silky-coated sheepdog with a long ruff and long narrow head developed in Scotland

sheep dog (noun)	collie (noun)
any of various usually long-haired breeds of dog reared to herd and guard sheep	a silky-coated sheepdog with a long ruff and long narrow head developed in Scotland