The complete Pokemon Diamond and Pearl pokedex, part 10

Pokémon Name: Cresselia
Type: Psychic
Classification: Crescent
Pokédex Number: 488 National
Ability: Levitate – Immune to Ground-type attacks
Useful Attacks: Psychic
Location Found:
D/P/P: Roaming Sinnoh after visiting Fullmoon Island
HG/SS: Trade with D/P/P
B/W: Poke Transfer

Cresselia is a swan-like, moony 'mon that's as graceful as it is bizarre. With a flexible moveset and above-average stats, focusing on defense, Cresselia is also the only legendary Pokémon to have a gender. Cresselia won't reveal itself to you until after you've defeated the Elite Four and become the Sinnoh Pokémon Champion. Once you've done all that, the mysterious Fullmoon Island appears in the Sinnoh sea, beckoning you to challenge it.