attack dog

plural attack dogs

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. A dog used in the bloodsport of dogfighting.
3. figurativelyA 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, Mark Jaffe, The Gilded Dinosaur, page 146

4. figurativelyA person or other entity who fights physically on behalf of another.

   1 more example

   In the middle of the night, snow falling all around, the Hound digs the family a grave. We didn’t need this episode to know the Hound has changed. Even if we hadn’t seen his relationship with Arya start to work its magic on him, the Hound’s time with the septon who saved him shows how far he’s come from the Lannisters’ attack dog.

   2017 July 16, Brandon Nowalk, “Chickens and dragons come home to roost on Game Of Thrones (newbies)”, in The Onion AV Club