The Pokémon TCG: EX Delta Species expansion was released on October 31st, 2005.
EX Delta Species
EX Delta Species gives you the opportunity to experience new adventures in a faraway land never before seen in the Pokémon universe! Researchers have built a city called Holon. The people of Holon study mysterious, magnetic powers in the area. The powers have affected the local Pokémon in ways that scientists have barely begun to understand. These new Pokémon are called Delta Species Pokémon because they are different from standard Pokémon, Pokémon-ex, and Pokémon "star." The people of Holon are hard workers, and their industriousness is reflected in the new Holon Supporter cards. And, as scientists begin to harness the power around Holon, new Energy cards become available... including Pokémon that can become Energy cards!
Delta, Delta, Delta!
The most obvious new feature of EX Delta Species is the introduction of Delta Species Pokémon. Delta Species Pokémon behave a lot like normal Pokémon: you can evolve a Delta Species Pokémon from a normal Pokémon (or a normal Pokémon from a Delta Species Pokémon), and they can use most of the Pokémon Tool cards, Special Energy cards, and other types of cards that normal Pokémon can use. However, several Delta Species Pokémon can be almost as powerful as Pokémon-ex when used well.
Metagross (11/113) is one example of an incredibly powerful Delta Species Pokémon. Its Crush and Burn attack is a devastating attack previously seen only on Electrode ex (EX FireRed & LeafGreen, 107,112) as Crash and Burn. It does 30 damage, plus 20 damage for each Energy card you wish to discard from any of your Pokémon. If you still think Metagross' attack wasn't frightening enough, take a look at Metagross' Delta Control Poké-Power that lets you look at the top four cards of your deck and choose one to put into your hand! You have to put the other three cards at the bottom of your deck afterward, but if you want one of them, you can play Professor Cozmo's Discovery (EX Deoxys, 90/107) for the chance to put those three cards into your hand. Dragonite (3/113) works well with Metagross, but then again, it works well with any deck that requires Lightning Energy cards! Dragonite's Delta Charge Poké-Power lets you attach a Lightning Energy card from your discard pile to one of your Benched Pokémon. Dragonite also has an Agility attack that does 30 damage, and if your opponent attacks Dragonite after Dragonite uses Agility, your opponent's attack fails if he or she flips tails!
Even though Salamence (14/113) is a Fire-type Pokémon, it has a Weakness to Colorless-type Pokémon, rather than the usual Water-type Pokémon. This is a very important distinction, since it means that it is now possible to have a deck of Fire-type Pokémon that are not vulnerable to Water-type Pokémon! In fact, Salamence can destroy many Water-type Pokémon with its Delta Blast attack. It does 100 damage, which instantly Knocks Out most Water-type Pokémon with one hit!
The Holon Research Tower (94/113) Stadium card can be an important addition to any deck containing at least one Delta Species Pokémon. All basic Energy cards attached to a Delta Species Pokémon may provide either their usual Energy type or Metal Energy, but only one at a time. That means that many problems with Metal Energy that Delta Species Pokémon might have are solved! Salamence, for example, must discard one Metal Energy when using Delta Blast. But if the basic Energy cards attached to Salamence can provide Metal Energy, that means you can just discard one of those instead! You won't have to discard an actual Metal Energy card.
Source:
https://www.pokemon.com/us/pokemon-tcg/ex-delta-species