You can’t get this guy any other way besides as an early purchase bonus.
The Pokemon Company
Nintendo and The Pokemon Company are offering an early purchase bonus for Pokemon Brilliant Diamond and Shining Pearl. Everyone who picks up a physical or digital copy of either Switch title before Feb. 21, 2022, is able to receive a free Manaphy egg in their game.
Manaphy is a water-type Mythical Pokemon — a rare monster that can’t be obtained through normal gameplay, making this early purchase offer the only confirmed way to get one in Brilliant Diamond and Shining Pearl. The egg will be distributed via the Mystery Gift function, which unlocks approximately two hours into the adventure.
After claiming the egg, you’ll need to walk a certain number of steps in the game with it in your party in order for Manaphy to hatch. If you then leave Manaphy together with a Ditto at the Pokemon Nursery in Solaceon Town, it will produce an egg that hatches into Phione — another rare Pokemon that is otherwise unobtainable in the games.
Pokemon Brilliant Diamond and Shining Pearl are set for release on Nintendo Switch on Nov. 19. The pair are remakes of the original Pokemon Diamond and Pearl versions for the Nintendo DS, featuring updated visuals and other gameplay enhancements.
A new open-world Pokemon adventure titled Pokemon Legends: Arceus is also coming to Nintendo Switch a few months later, on Jan. 28. It, too, has an early purchase bonus. Everyone who picks up the game before May 9, 2022, can receive a Hisuian Growlithe kimono outfit for their player character.
Source from www.cnet.com