Legendary Pokémon
Giratina is a Ghost/Dragon type Pokémon with two forms- an Altered Form, which is its form in most cases, and has higher defensive statistics, and an Origin Form, which is its form if it is in the Distortion World, or holding a certain item. Due to this item and the Distortion World not existing in Pokémon Diamond and Pearl, a Giratina in those games can only be in Altered Form.