The Pokemon Go Plus is an accessory that was made specifically for use with the Pokemon Go mobile game. It can only be connected to a Bluetooth enabled smartphone and only works with the Pokemon Go mobile game.
Nintendo has announced a similar peripheral for Pokemon Let's Go Pikachu and Pokemon Let's Go Eevee, however. A special controller called the Poke Ball Plus will be released alongside these new Pokemon games for the Nintendo Switch. The Poke Ball plus will be available in a special edition bundle with either game, or separately.
The Poke Ball plus can be used to capture Pokemon in the new Switch games, to carry Pokemon around with you in the real world while not playing on your Switch. It will also be able to connect to Pokemon Go to allow you to capture Kanto region Pokemon from Pokemon Go to transfer directly to Pokemon Let's Go Pikachu or Eevee.
The Poke Ball Plus is a similar device to the Pokemon Go Plus, but with additional features and can be used with the upcoming Nintendo Switch games. It also allows connectivity between the Nintendo Switch games and the mobile game.