Pokémon Brilliant Diamond and Brilliant Pearl give you two Mythical Pokémon of your own. Gift Pokémon are a staple of the Pokémon series, and few are as sought after as Mew or Jirachi. You will need to defeat Roark and get the Coal Medal from Oreburgh City's Rock-type Gym. From there, head to Floaroma Town.
If you own Pokémon: Let's Go, Pikachu! o Pokémon: Come on, Eevee! save the data on your Nintendo Switch, the Mythical Pokémon Mew will be yours. There is a woman in Floaroma Town standing in a field of flowers. He talks to her and she will give you Mew.
Mew is a pure Psychic-type who can learn any move in the entire game. Feel free to get creative with the moves to use.
For trainers with save data on their Nintendo Switch from Pokémon Sword or Pokémon Shield, you can get the Mythical Pokémon Jirachi as a gift. There is a man standing next to the woman who has Mew in Floaroma Town. He will be in the field of flowers; talk to him and add the Wish Pokémon to your team.
Jirachi comes with the Serene Grace ability, which doubles the chance of a side effect of a Pokémon's attack. Its Steel / Psychic type makes it great for in-game and competitive teams. He experiments with moves, but you won't go wrong with Iron Head.