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 the Johto region. It is located between Mahogany Town and Blackthorn City.
It is similar to the Battle Frontier but much smaller. The park has a series of powerful, rental Pokémon which visiting Trainers can battle against. The park's 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.