Coming exclusively to Nintendo DS on March 22, 2009, Pokemon Platinum Version introduces players to a new world to explore in the Sinnoh region: The Distortion World. Many other surprises await, including the appearance of Giratina in its Origin Forme, the Pokemon's true Forme.
Giratina Origin Forme doesn't appear in Pokemon Diamond Version or Pokemon Pearl Version, so if you want to catch a glimpse of this Pokemon, you'll have to play Pokemon Platinum Version. The Forme of Giratina players encounter in Pokemon Pokemon Diamond Version and Pokemon Pearl Version is known as Giratina Altered Forme.
Source:
http://www.pokemon.com/