Here's why:
* Mewtwo's location in the original games: Mewtwo is found in the Cerulean Cave in Pokémon Red, Blue, and Yellow, not the "Monean Chamber".
* No Cerulean Cave in FireRed/LeafGreen: FireRed and LeafGreen versions have a different story and map layout. The Cerulean Cave, where Mewtwo is located in the original games, is not present in these versions.
To encounter Mewtwo in the original Pokémon games, you need to:
1. Complete the game: Beat the Elite Four and become the champion.
2. Travel to Cerulean Cave: Located in the north-western part of the Kanto region.
3. Explore the cave: Mewtwo is found deep inside the cave.
However, if you are playing FireRed or LeafGreen, you cannot catch Mewtwo within the game. You would need to use an external method like:
* Trading: You can trade for a Mewtwo from someone who has one.
* Events: Some events in the past allowed players to receive Mewtwo through special codes.
* Cheats: You can use cheat codes to add a Mewtwo to your game.
Remember, using cheats or unauthorized means to obtain rare Pokémon can compromise the integrity of the game.