sled dog vs working dog

sled dog and working dog both are nouns.

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

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

sled dog: a dog trained to draw a sled usually in a team
working dog: any of several breeds of usually large powerful dogs bred to work as draft animals and guard and guide dogs

sled dog (noun)	working dog (noun)
a dog trained to draw a sled usually in a team	any of several breeds of usually large powerful dogs bred to work as draft animals and guard and guide dogs