bull mastiff vs working dog

bull mastiff and working dog both are nouns.

Word	Noun	Adjective	Verb	Adverb
bull mastiff	Yes	No	No	No
working dog	Yes	No	No	No

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

bull mastiff: large powerful breed developed by crossing the bulldog and the mastiff
working dog: any of several breeds of usually large powerful dogs bred to work as draft animals and guard and guide dogs

bull mastiff (noun)	working dog (noun)
large powerful breed developed by crossing the bulldog and the mastiff	any of several breeds of usually large powerful dogs bred to work as draft animals and guard and guide dogs