Which Pokémon is the only one that can be encountered in the Distortion World? Giratina is the only Pokémon encountered there in Pokémon Platinum.
The Distortion World is an unstable alternate dimension introduced in Pokémon Platinum. It has no conventional wild-Pokémon areas: the player travels through floating platforms, inverted landscapes, puzzles, and passages where gravity and perspective behave strangely. The area is central to the game’s climax against Team Galactic.
Giratina appears at the end of the Distortion World as the story’s legendary encounter. After the player reaches it, Giratina can be battled and caught, making the location especially associated with its role as the ruler or inhabitant of that dimension. Its Origin Forme is also displayed there before the player eventually receives the Griseous Orb.
Dialga and Palkia are closely connected to the story and appear in later events, but they are not the Pokémon encountered as the Distortion World’s resident legendary. That distinction is why Giratina is the fair single answer.