The Mythical Pokémon Shaymin is set to appear in Pokémon Legends: Arceus and players can even catch it, and add it to their team. There is more to unlocking Shaymin in Pokémon Legends: Arceus than meets the eye, as it requires the player to have something that is created outside of the game.
Shaymin debuted in Pokémon Diamond and Pearl. It's a Grass-type Pokémon when it's in its Land Forme and Grass/Flying when in its Sky Forme. The player can use the Gracidea Flower on Shaymin to its Sky Forme, though it will revert back to its Land Forme at night or if inflicted with the frozen status condition. The existence of Shaymin was meant to be a secret, but fans discovered it early, thanks to a glitch in the Japanese version of Pokémon Diamond and Pearl, where the player could enter the door in Aaron's room, allowing them to break out of bounds. Years later, a brand new glitch in Pokémon Brilliant Diamond and Shining Pearl gave fans early access to Shaymin once again.
Pokémon Legends: Arceus is set in the Hisui region, which is the ancient version of the Sinnoh region from Pokémon Diamond and Pearl. As such, there are Mythical and Legendary Pokémon that were introduced in Gen IV that will be making an appearance in Pokémon Legends: Arceus. It's expected that Arceus will have a central role in the story, considering its name is in the title of the game, but there are other Mythical Pokémon from the same generation that are set to return, including Shaymin.
How To Unlock Shaymin's Mission In Pokémon Legends: Arceus
There are two requirements for the Shaymin (Land Forme) encounter in Pokémon Legends: Arceus. The first and most crucial is that the player needs save data from Pokémon Sword and Shield to unlock the encounter in the first place. This is similar to previous save unlock bonuses in the Pokémon games on the Nintendo Switch, such as the Gigantamax Eevee and Pikachu in Pokémon Sword and Shield requiring data from the corresponding Let's Go game. The player doesn't need to have completed Pokémon Sword and Shield to unlock Shaymin, as save data from any point in the game (including the first few minutes) should suffice. This save data will also unlock the Shaymin Kimono clothing set, which can be equipped within the first few hours of gameplay.
Like many Mythical and Legendary Pokémon throughout the series, Shaymin can only be encountered once the player completes the game and witnesses the credits roll. If they have save data from Pokémon Sword and Shield, then a new mission will become available in Jubilife Village, allowing the player to encounter and possibly capture Shaymin.
Pokémon Legends: Arceus will be released for Nintendo Switch on January 28, 2022.