Here's why:
* Redgirus is from Pokémon Ruby, Sapphire, and Emerald. These games have a different region (Hoenn) and a completely separate storyline compared to Diamond, Pearl, and Platinum.
* Regigigas, the Pokémon you're likely thinking of, is obtainable in Diamond, Pearl, and Platinum. However, you can't get it until you've already beaten the game and have access to the post-game content.
Here's how to find and catch Regigigas in Diamond, Pearl, and Platinum:
1. Complete the main story and defeat the Elite Four.
2. Visit the Snowpoint Temple in Snowpoint City. This is the temple you enter to get to the legendary Pokémon, Regice, Regirock, and Registeel.
3. Catch the three legendary Pokémon:
- Regice is found inside the temple.
- Regirock is in the cave to the east of the temple.
- Registeel is in the cave to the west of the temple.
4. Return to the Snowpoint Temple with all three caught. A giant door will open, and you'll be able to enter the deeper part of the temple.
5. Battle and catch Regigigas. He's a very powerful Pokémon, so be prepared for a tough battle.
Let me know if you have any other questions about Pokémon Diamond, Pearl, or Platinum!