The Grass-type Legendary Pokémon Shaymin was first introduced in Pokémon Platinum, but was unattainable through normal gameplay. You had to use the Mystery Gift feature to receive an exclusive event item that would later make the little Pokémon look like a hedgehog. Ahead of the launch of Pokémon Brilliant Diamond and Brilliant Pearl, a burning question in the community was whether Shaymin, among other event-only Pokémon, would be obtainable by other means, if at all. If recent datamines are anything to rely on, it turns out that Brilliant Diamond and Shining Pearl are pretty loyal to the games they are based on, right down to their unique in-game events.
The dataminers have discovered Shaymin within the files of Brilliant Diamond and Shining Pearl and it seems that it will be acquired with the exact same method of over a decade ago: through a mysterious gift to be distributed at a later time. The footage of the event, including the Seabreak Path and the Flower Paradise where Shaymin resides, seems to confirm that the event actually exists, it's just a matter of when Nintendo will allow us to access it; at the time of writing, it's not entirely clear when that will be.
In Pokémon Platinum, after receiving your Secret Gift, a postman in a Poké Mart will deliver you a letter from Professor Oak from the Kanto region asking you to meet him at Route 224. After talking to him, a linear path opens up north, taking you to Shaymin. It seems likely that you will have to follow the same steps to get it in remakes when the event breaks down in the future.