attack dog

Short definition from Wiktionary

Noun

1. 1
   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.
2. 2
   A dog used in the bloodsport of dogfighting.
3. 3
   A person who engages in rhetorical attacks on behalf of another.
4. 4
   A person or other entity who fights physically on behalf of another.