Shaymin, the half-shrub, half-hedgehog legendary Pokémon, has been catchable in Pokémon Brilliant Diamond & Shining Pearl since 27th February 2022.
All you need to do is open the game, receive the "Oak's Letter" item through the Mystery Gift feature (unlocked after you've played at least 2 hours of the game, no Nintendo Switch Online membership needed) and then you'll be able to catch Shaymin after you've entered the Hall of Fame and obtained the National Pokédex, which you can get by completing the National Pokédex and talking to Professor Rowan.
Oak's Letter, which is the important part of this equation, will only be available until Sunday, March 27th, at 2:59pm UTC / 9:59am EST / 6:59am PST.
Have you got a Shaymin? What's his name? Let us know in the comments!