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Basic info on Delta Species:
>Delta Species Pokémon are of a different type than the one typically seen for that species of Pokémon
>Energy Cost for moves match that of their new type
>Weakness and Resistance do not change from their old type(s)
>Lacks a change in appearance aside from an aura of the color matching the new type, although not all cards featuring Delta Species Pokémon have this
>If the Delta Species has 2 types (for TCG), the secondary type is always Metal and is almost always fully evolved
>Delta Species doesn't affect evolution (in terms of TCG gameplay), e.g. A Pokémon can start out as Delta, evolve into a regular variant, then evolve into Delta again
>There's no known restriction to which or how many types a Pokémon can change to as a Delta Species, e.g. Delta Horsea have been seen as either Fighting or Fire
To correlate with later introduced terms, Delta Species can be seen as a mix between regional variants and Terastal.