Legendary Pokémon

Legendary Pokémon are a group of incredibly rare and often very powerful Pokémon, generally featured prominently in the legends and myths of the Pokémon world.

Mythical Pokémon are a related but separate group of Pokémon, which are usually event-exclusive. In Western language media, Mythical Pokémon were considered to be Legendary Pokémon prior to Generation V; in Japanese and Korean media, the two groups have always been considered distinct.

No explicit criteria defines what makes a Pokémon a Legendary Pokémon. Instead, the only way to identify a Pokémon as belonging to this group is through statements from official media, such as the games or anime.

In game
There are currently four Legendary Pokemon, each in their own specific region (Mewtwo in Kanto, Rayquaza in Hoenn, Arceus in Sinnoh, Keldeo in Unova). While they are obtainable, they cannot be kept permanently. If one is caught, it will replace the last Pokémon in the player's party and a notofication will appear to other players in the same region.

They are encountered at level 100 and have max EVs and IVs.

They appear as followers behind the current owner and cannot be unsummoned or switched. The player holding the Legendary Pokémon will be forced to accept duel challenges and upon defeat, the Legendary will be transferred to the victor.

Having a Legendary Pokémon in your team will prevent travelling to other regions or switching channels, or using escape moves or items, such as Fly or Escape Rope. Disconnecting from the game or depositing it to the PC box will cause to Pokémon to flee and return to the wild. They will also return to the wild by themselves, after some time.

Exceptions
Zekrom in Unova is currently the only exception to this, as it must be caught in order to defeat N and Ghetsis. If the player fails to catch it, they will be warped back and can try again. After finishing this storyline, Zekrom will go to the player's room.

Giratina in Sinnoh is a boss battle and is unobtainable.