working dog

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A dog of any of various breeds developed or trained to do useful work, such as herding animals, pulling wagons or sleds, or guarding property.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Any breed of dog developed primarily to assist humans in performing tasks, such as farming, controlling livestock, law enforcement, or assisting the blind.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun any of several breeds of usually large powerful dogs bred to work as draft animals and guard and guide dogs