From Bulbapedia, the community-driven Pokémon encyclopedia.
Battle Park (Japanese: バトルパーク Battle Park) is an anime-exclusive battle facility in Johto. It is located 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. Ash, Misty and Brock where advised to go here by an unnamed Trainer.