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According to the February issue of CoroCoro Comics, an additional Pokémon star of the 10th movie, Dialga vs. Palkia, will be revealed. Pokeani reports that it is likely to be one of three event-only Pokémon from Diamond and Pearl- Darkrai, Sheimi or Arseus.
The name of the additional Pokémon may or may not be added to the movie's title. More information will be provided closer to the magazine's release.