If you’re curious about whether or not Shaymin is in Pokémon Legends: Arceus, the good news is that yes, it is. You’re also able to alternate between Shaymin’s Land and Sky Formes at will, meaning you’ve got the option of using it as a pure Grass-type or as a Grass/Flying hybrid. You just have to catch it first.
Triggering Legends: Arceus‘ Shaymin encounter has quite a few prerequisites, including ownership of a previous game, reaching a certain point in the main story, and more. You can’t just waltz into the first area of Hisui and expect to see a Mythical Pokémon hanging out next to a level 2 Bidoof.