https://www.gamespot.com/articles/e3-2018-legendary-pokemon-mew-is-an-exclusive-with/1100-6459710/

While we already knew about the Poke Ball Plus, a Joy-Con controller for Nintendo Switch that’s due to be released alongside Pokemon Let’s Go Pikachu and Eevee, Nintendo announced during its E3 presentation that every Poke Ball will come with an exclusive Pokemon: The legendary Mew.

While the controller can be used to play the Pokemon Let’s Go games, it can also be used to “store” a Pokemon from those games and carried around with you, and you’ll earn rewards when you do. This is similar to the Pokewalker pedometer accessory from Pokemon HeartGold and SoulSilver.