From Bulbapedia, the community-driven Pokémon encyclopedia.
Battle Park (Japanese: バトルパーク Battle Park) is a facility in northern Johto between Mahogany Town and Blackthorn City. It is similar to the Battle Frontier of Hoenn but much smaller. The park has a series of powerful, random rental Pokémon Trainers can battle against. The rental Pokémon all know unique moves, and are trained to only be obedient while inside the park, and to return immediately if they are lost or stolen. It was first mentioned and debuted in One Trick Phony! by an unnamed Trainer, where Ash was to battle a Venusaur, Charizard, and Blastoise.