attack dog

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

• noun Any dog bred; trained or used for the purpose of attacking a target, either on command or on sight; trained either as a specialized form of police dog or a security force dog.
• noun A dog used in dogfighting; a bloodsport.
• noun A person who engages in rhetorical attacks on behalf of another.

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

• noun a watchdog trained to attack on command