The complete Pokemon Diamond and Pearl pokedex, part 9

Pokémon Name: Mesprit
Type: Psychic
Classification: Emotion
Pokédex Number: 147 Sinnoh / 481 National
Ability: Levitate – Immune to Ground-type attacks
Useful Attacks: Natural Gift
Location Found:
D/P/P: Roaming Sinnoh after visiting Verity Cavern (Lake Verity)
HG/SS: Trade with D/P/P
B/W: Poke Transfer

Mesprit lived beneath Lake Verity, a lake adjacent to your Trainer's hometown of Twinleaf. As such, it seems that Mesprit feels a certain affinity for your trainer, and thus decides to play with you by running running about the entirety of the region rather than patiently wait to be captured in the cave at the bottom of the lake like its brothers. Naturally, as with Gold and Silver's Legendary Beasts and Ruby and Sapphire 's Latios and Latias, this is a colossal and irritating undertaking, requiring some speedy Pokémon with a move that can put Mesprit to sleep. Mesprit's stats are solid and the most balanced of the lake guardians, making it a jack of all trades.