Mesprit, along with Azelf and Uxie, is a Lake Guardian found in Pokemon Diamond. While all three Lake Guardians are found in the Sinnoh region, each game features only one of the three.
Here's why:
* Version Exclusives: In the Diamond, Pearl, and Platinum games, the Lake Guardians were divided as version exclusives to encourage trading and completing the Pokédex.
* Diamond: Features Azelf, Mesprit, and Uxie.
* Pearl: Features Uxie, Azelf, and Mesprit.
* Platinum: Features all three Lake Guardians, but requires you to complete specific events and conditions to catch each one.
To catch Mesprit, you'll need to trade with someone who has a copy of Pokemon Diamond.