Mewtwo is a legendary Pokemon, and as such, it's not available in the main game. You may be thinking of the following:
* Special Events: Some games had events where you could get Mewtwo, but these were limited time events.
* Cheats: Using cheat codes or save file editing can allow you to get Mewtwo, but this is outside of the normal game play.
* Fan-made Modifications: Some players create their own modifications for the game, which can include adding Mewtwo.
However, you can find Mewtwo in Pokemon Mystery Dungeon: Explorers of Darkness by transferring your save file from Explorers of Time, a feature only available in the Japanese version of the games. This allows you to obtain Mewtwo as a partner by completing a specific side quest.
Remember: If you are looking for Mewtwo in Pokemon Mystery Dungeon, it's important to be aware of where you can and cannot get it.