Shaymin, the 492nd Pokemon, is the newest mysterious Pokemon. Like Manaphy and Darkrai before it, Shaymin does not appear in normal gameplay in Pokemon Diamond and Pearl. It is said that in some regions, it is customary to give a Shaymin-shaped flower bouquet as a thank-you gift. Shaymin stars in the next Pokemon movie. How do you get Shaymin in your Pokemon Diamond or Pokemon Pearl game?

Shaymin's Exclusive Move is Seed Flare!
Seed Flare is a move that only Shaymin can use. Generating a shockwave from Shaymin's body, Seed Flare has a staggering 120 Power and occasionally lowers the foe's Special Defense.

Shaymin's Ability is Natural Cure! Shaymin has a very useful Ability called Natural Cure. Natural Cure restores the Pokemon status conditions (such as Sleep or Poisoned) which the Pokemon is withdrawn from battle, or after the battle ends.

Source: http://www.pokemon.com/