Cresselia is a Pokemon exclusive to Pokemon Pearl.
Here's why:
* Version Exclusives: In Pokemon Diamond and Pearl, there are specific Pokemon that can only be found in one version or the other. This is a common feature in the Pokemon series to encourage trading between players.
* Cresselia's Role: Cresselia is the counterpart to the legendary Pokemon Darkrai, which is found in Pokemon Diamond.
To get Cresselia, you would need to trade with someone who has a Pokemon Pearl game.
Let me know if you have any other Pokemon questions!