Delta Pokémon are a special kind of Pokémon whose type and appearance differs from their normal counterpart.
In the game
Shortly after the events in the Cult of Darkrai base, the player decides to start a Pokémon journey and seeks out Professor Sylvan to obtain a Starter Pokémon. Upon entering Professor Sylvan's lab, the player interrupts a conversation between Reukra and Professor Sylvan about Delta Pokémon. Reukra suggests that the player and Damian take the Delta Pokémon as starters.
There are other Delta Pokémon the player may encounter throughout their journey. It is not known if these Pokémon have been altered artificially or through a natural mutation, but it is hinted by Reukra that some may be the result of experiments led by the Cult of Perfection in their search for a perfect Pokémon.
Delta Pokémon can only breed with Delta Ditto, with the exception of Delta Ditto itself, which can breed with any Pokémon that is not in the Undiscovered Egg Group, or Ditto Egg Group. Additionally, Delta Pokémon whose non-Delta counterparts are in the Undiscovered Egg Group (Delta Pokémon whose base forms are baby, legendary, or mythical Pokémon) are unable to breed.
The weight of a Delta Pokémon is either equal to the base form's weight or equal to half or double the base form's weight. With the exception of the Delta Bulbasaur and Delta Combee evolutionary lines, Delta Pokémon have the same gender ratio as their non-Delta counterparts. All properties of Delta Pokémon that are not mentioned in this section are the same as that of their non-Delta counterparts.