Boom-Boom was Mad Stan's pet Chihuahua.
Boom-Boom greeted Stan as he came home, and noticed that he had bought an unconscious being with him. Stan explained that he was another victim of an uncaring society, but when Boom-Boom got close to him, the being was revealed to be Zeta, who had been shut down. Believing that something was being plotted against him, Stan attached a bomb to Zeta, reactivated him, and set him out into the open. Stan then recorded a message on a disk and attached it to Boom-Boom's collar in the event Batman find the place. Sure enough, he did and found that Stan had sedated himself so that he wouldn't be able to give any information. Boom-Boom approached Batman rather fiercely as he played the message, which said he had till midnight to find Zeta and the bomb.