What is the mechanical hound and what is its purpose ?

The Mechanical Hound is an android-like dog that tracks the scent of its prey with a vengeance. It also hunts illegal books or anything else that the state deems necessary. The Hound injects a poison into their victim which incapacitates them. The Mechanical Hound attacks Montag, and Montag destroys it with the flamethrower, but not before it stabs him with a needle full of anesthetic.