From Bulbapedia, the community-driven Pokémon encyclopedia.
δ Delta Species (Japanese: δ-デルタ種 δ-Delta Species) are a special kind of Pokémon found in the TCG. The term is often shortened to δ, DS, or Delta. They first appeared in the EX Delta Species expansion, and were last released in the EX Dragon Frontiers expansion.
According to the backstory offered in EX Delta Species, Pokémon become δ Delta Species upon exposure to electromagnetic waves, which were produced by scientists in Holon attempting to locate Mew. Why the scientists chose to use Greek letter δ is debated. While it appears to come from the scientific use to denote a change, the uppercase Δ would have been more appropriate, as it determines a major change. Another common theory is that Delta is the fourth letter of the Greek alphabet, and δ Delta Species are the fourth sort of Pokémon after normal Pokémon, Pokémon-ex, Pokémon Star, and Dark Pokémon in the EX series. Another possibility is that Delta Species has the same initials as the Nintendo DS, which was recently released at the time.
δ Delta Species are different than normal Pokémon, as their type is not what they would be expected to be. For example, Rayquaza δ becomes a dual-type Lightning/Metal Pokémon, as opposed to a Colorless-type. Dual type Pokémon δ always have Metal as their secondary type, and it is nearly always fully evolved Pokémon or legendary Pokémon that display this. However, with the release of EX Dragon Frontiers, no Pokémon δ displayed this Metal dual type, leading some to believe that these Pokémon became δ Delta Species naturally or the trait was outbred on the island habitats in which they live.
In EX Delta Species, if a Delta Species Pokémon had one of its types changed to something other than Metal, it was always either to Fire or Lightning. This is likely due to the fact that, since these two types are based upon one of the game types each, whereas others are based on more, all cards of those two types shared a single weakness among them: Water and Fighting, respectively. Thus, Fire Pokémon and Lightning Pokémon became more playable by diversifying their weaknesses. Later sets did not do this, however, and Delta Species Pokémon could be of any type.
In terms of card design, δ Delta Species have a coil design going around their image border, and the evobox circle becomes a large letter δ; Basic Pokémon have an empty letter δ.
The designation of δ Delta Species does not affect the name of a Pokémon, e.g. a normal Dragonite can evolve from a Dragonair δ and vice versa. If a card wishes to reference δ Delta Species, it uses the text "Pokémon that has δ on its card."
List of expansions that contain δ Delta Species
Pokémon found as δ Delta Species