Quite a few of the rarest and most unique Pokemon are no longer found directly within the games themselves. However, they are released periodically by way of promotional downloads. The latest of these is actually a retread: Deoxys.
With four forms, this alien Pokemon is extremely versatile. Thus far, it has only been available at promotional events, never appearing natively in a Pokemon game. The trend continues when Deoxys graces the Nintendo Wi-Fi Connection again from May 8 to May 31, available for Pokemon Black 2 and White 2.
Deoxys, in this case, will be able to shift between all four forms (Normal, Attack, Defense, and Speed) and will already be at level 100. As of yet, the special moves available to this version of the DNA Pokemon have not yet been revealed.
Source: IGN