Pokemon Legends: Arceus will offer free bonus creatures and cosmetics to long-term fans of the series.
The game's official website confirms that "if you have play records from the Pokemon Sword or Pokemon Shield game, you'll be able to take on a research request in Pokemon Legends: Arceus in which you'll have a chance to add the mythical Pokemon Shaymin to your team."
The quest won't be available until after you've seen the end credits, but you'll still be able to explore the Hisui region with the adorable hedgehog pokemon at your side. It sounds as though Shaymin's hog-based 'Land Forme' will be the only version available, and that you might not have access to its alternative 'Sky Forme.'
As well as Shaymin itself, you'll also unlock a cosmetic set inspired by the Pokemon, which you can pick up from the clothier after you join the Galaxy Expedition team, about an hour into the game.
Similar bonuses will be available if you've got save data from Let's Go Pikachu or Let's Go Eevee on your system. Those games will offer either a Pikachu or Eevee mask, which you can unlock from the clothier at the same point in the game.
It sounds as though as long as you have save data for the games somewhere on your Switch, you should get automatic access to the bonus content, even if you don't have the games themselves actually installed when you start playing.
If you're planning to catch up before Arceus, here's our list of the best Pokemon games.