The Tao Trio
The Tao Trio, also known as the Energy Trio, contains three Legendary Pokemon: Zekrom, Reshiram and Kyurem. Zekrom is the version mascot of Pokemon White; it appears on the box art for the game and may only be caught in it. Similarly, Reshiram is the version mascot of Pokemon Black, appearing on the box art from the game and only available within it. Kyurem, the third member of the trio, can be caught in either version of the game. If you want to have Zekrom in Pokemon Black or Reshiram in Pokemon White, you will have to trade for them with another player.
The Musketeer Trio
The Musketeer Trio is comprised of Cobalion, Terrakion and Virizion, three Pokemon based on the characters from Alexandre Dumas' novel "The Three Musketeers." These Pokemon share their secondary type, the Fighting type, as well as their signature move, Sacred Sword. All of them may be caught in both Pokemon Black and Pokemon White.
The Kami Trio
The Kami Trio, or God Trio, consists of Tornadus, Thundurus and Landorus, three Pokemon based on three different Japanese deities. This trio is similar to the Tao Trio in that one of its members, Tornadus, may only be caught in Pokemon Black, another, Thundurus, may only be caught in Pokemon White. The third member, Landorus, may be caught in either Black or White, but will only appear if you have both Tornadus and Thundurus in your party. You will have to trade with another player to obtain both the Pokemon unavailable in your game version and Landorus.
Mythical Pokemon
In addition to the Legendary Pokemon, the fifth generation also introduced several new Mythical Pokemon. Although in the English version of the games they are classified as Legendary Pokemon, in the original Japanese version they belong to a different category. These Pokemon are restricted to official Nintendo events and may not be obtained during normal game play. Fifth-generation Mythical Pokemon include Victini, Keldeo, Meloetta and Genesect.