Ronny Boxer operated illegal dog fighting rings.
For an unknown amount of time, Boxer purchased puppies to raise as fighting dogs and organized dog fights from the backrooms of shady joints, abandoned warehouses and barges in the docks of Gotham. One of these dogs grew up to be Ace. Boxer was eventually arrested once during Ace's debut fight, where the hound escaped in the chaos and wound up in the care of Bruce Wayne.
Years later, Boxer continued his illegal dog fights, going several steps further in his enterprise. He completely mechanized his operations, removing the need for animal handlers and broadcasters, choosing to broadcast his dog fights over the net. He also engaged in dabbling with chemistry, making his own supply of the experimental growth hormone Cerestone, which he used to forcibly enhance his dogs into monstrously grown canines. His first experiment mutated into a horrific monster the size of an elephant.
Boxer eventually came across Ace again while Terry McGinnis took him out on a walk. Ace, remembering Boxer, chased after him. Boxer managed to escape while Ace ran away from Terry. Shortly after, Boxer found Batman in his home, performing investigations and discovering his chemical laboratory. In the fight that ensued Boxer's home blew up, but he managed to escape once again.
Hours later, Boxer went on to hold his new dog fight, located on a ship where the dogs were held and the fights would be held offshore. By chance, one of his cronies, Maddie, had captured Ace earlier and torn off his tag, attempting to sell him to Boxer. He recognized Ace this time, musing that perhaps fate had brought him and the angry dog together again.
Led by a tip from Bruce, Batman tracked Boxer down. Boxer trapped Batman in his fighting arena and set his monstrous bulldog on him. As Batman and Ace dealt with the beast, Boxer tried to make a run for it, but by then the ship had sailed out to sea. Ace followed and attacked, leaving him dangling over the edge of the ship. Boxer was later presumably arrested.