Celebi

Celebi (Japanese: セレビィ Celebi) is a /-type Mythical Pokémon first introduced in Generation II. It was the last Pokémon in the National Pokédex until Generation III was released. It can only be caught in Pokemon Crystal by using the GS Ball, which can only be obtained by an event.

Pokedex entries
It will help you get zoura in pokemon white and black.

Pokémon Mystery Dungeon
A pink Celebi joins the players group in the world of the future, in the last dungeon before returning to the past. She has an unusual personality, and is an old friend of Grovyle, helping him go to the past twice. In the game, it is hinted that she may have feelings for Grovyle. Celebi will only join your team for a short period to reach a place in the main story, and can't be seen again unless you play the Special Episode "In The Future Of Darkness." In the Special Episode Celebi has been kidnapped by Spiritomb, and will only be on your team for the last Dungeon in the special.

Super Smash Brothers
Celebi is also in Super Smash Bros. Melee and Super Smash Bros. Brawl. It is rare to unleash, and will only come out of a Poké Ball, like all Pokémon. Once Celebi is released, it drops down trophies while it flies upward toward the top of the screen until it disappears. You can also get the Celebi trophy by completing certain tasks in the game.

Pokémon 4Ever
In the Pokémon movie Pokémon 4Ever, it is the star/main Pokémon, along with Sucuine. It inadvertently takes Sam from the past while trying to flee from a bounty hunter only to end up in the sights of a more sinister hunter.

Zoroark: Master of Illusions
Celebi also played a role in the 13th movie as Zorua's friend.

Trivia

 * Celebi was the second Pokémon introduced who is not legitimately obtainable in any main series game as of now, the first being Mew.
 * It is possible to obtain Celebi in the Japanese version of Crystal, by placing the GS Ball on the Shrine in Ilex Forest.
 * Celebi bonded with a younger version of Professor Oak in Pokémon 4Ever. In the same movie, Celebi is revealed to be able to take people and pokemon into the past or future.
 * Celebi could be obtained on the Pokémon Store on New York that opened November 16, 2001. During the first month of opening you could put your game into their PokéDex and you would get a Celebi.
 * Celebi is the only Pokémon thus far that is able to time travel, other than Dialga, the Time Deity.
 * In HeartGold and SoulSilver, when you approach the shrine in Ilex Forest with a promotional Celebi, you will go back in time 3 years to battle Giovanni.
 * The only way to obtain a Shiny Celebi outside of Japan is through hacking or the Gamestop/GAME Event.
 * A hacked Celebi will not trigger the Zorua event in Black and White. Only Celebi from the GameStop/GAME event will have the bond with the Zorua in Castelia City. This also resembles the part in the thirteenth movie, where Celebi shared great bond with Zorua.
 * In PokéPark Wii: Pikachu's Adventure after you beat the game in the Meeting Place, there's a sign with a code on it that when you enter it, you can encounter Celebi.
 * Celebi can be considered the rarest pokemon of all, as Mew can be caught at Pokemon Ranch as well as through the Mew Glitch, Deoxys can be caught by event for all the Gen III games, Jirachi can be obtained in all three gen II games as well as in Heartgold Soulsilver events, Regigigas can be obtained by migrating the three regis into the first three gen IV games which can be obtained without the aid of events in Ruby, Saphire and Emerald, Darkrai can be caught in the first three gen IV games by aid of event, Manaphy can be caught in all of the gen IV games with an event, Shaymin can be caught in the first three gen IV games by aid of event, Arceus can be caught in the first three gen IV games and the gen V games by aid of events, Victini can be caught in two different events and Meloetta, Keldeo and Genesect aren't technically official pokemon and Celebi can only be caught in gen II, in one game version (Crystal) in only one language (Japanese Crystal game) with an event.