Celebi

Celebi (セレビィ Celebi) is a powerful /-type Mythical Pokémon first introduced in Generation II. It was the last Pokémon in the National Pokédex until Generation III was released.

Apperence
Celebi is a green pokemon with large blue eyes and it has blue tipped antenna. It has a small pair of wings and light green arms and small darker green legs. There is almost always a smile on Celebi's face.

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

Pokémon Mystery Dungeon
A crystal/shiny 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.

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 Pokéball, like all Pokémon. Once Celebi is released, it drops down trophys 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.

Trivia

 * Celebi was the second Pokémon introduced who is not legaly obtainable in any game as of now, the first being Mew. However, in Pokémon Heart Gold/Soul Silver, it is possible to obtain one.
 * Celebi is the only Pokémon thus far that is able to time travel, other than Dialga, the Time Deity.
 * Celebi has a site that is dedicated to it, as it shares its Japanese name, Serebii.net.
 * Celebi is in Pokemon Movie 13.
 * Celebi, along with Dialga can time travel.
 * Celebi is along with most other legendary pokemon can only be obtained through an event.
 * There will be a Celebi event for Pokemon Heart Gold and Soul Silver starting from February 21st - March 7th.