The official Pokémon Legends: Arceus website has announced that players of Pokémon Sword game or the Pokémon Shield game can get the Mythical Pokémon Shaymin (Land Forme) and Shaymin Kimono Set if they have a save on their system. Players of Pokémon: Let’s Go, Pikachu! game or the Pokémon: Let’s Go, Eevee! game can also get Pikachu and Eevee-Themed Masks if they have a save game.

Special Bonuses

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Catch the Mythical Pokémon Shaymin (Land Forme)
If you have play records from the Pokémon Sword or Pokémon Shield game, you’ll be able to take on a research request in Pokémon Legends: Arceus in which you’ll have a chance to add the Mythical Pokémon Shaymin (Land Forme) to your team.

This research request will be available for you to accept in Jubilife Village after viewing the game’s end credits.

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Shaymin Kimono Set
You’ll also be able to claim the Shaymin Kimono Set. You can claim this in-game outfit by speaking to the clothier after joining the Galaxy Expedition Team. (It takes approximately one hour of play time to reach the point where this option is available.)

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Get Pikachu- and Eevee-Themed Masks
If you have play records from either the Pokémon: Let’s Go, Pikachu! or Pokémon: Let’s Go, Eevee! game, you’ll be able to claim both the Pikachu Mask and the Eevee Mask in Pokémon Legends: Arceus.

You can claim these masks by speaking to the clothier after joining the Galaxy Expedition Team. (It takes approximately one hour of play time to reach the point where this option is available.)

Source: https://legends.pokemon.com/en-us/shop/bonus-3/