attack dog

noun4

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. •
   (figurative) A person who engages in rhetorical attacks on behalf of another.

   • It was, however, as Charles Darwin's alter ego, an attack dog for the theory of evolution, that Huxley gained his greatest notoriety. — 2001
3. •
   (figurative) A person or other entity who fights physically on behalf of another.
4. •
   A dog used in the bloodsport of dogfighting.