Celebi

Celebi (セレビィ Celebi) is a /-type Legendary Pokémon first introduced in Generation II. It was the last pokemon in the Pokedex until Generation III was released

Pokémon 4Ever
In the Pokémon movie Pokémon 4Ever, it is the star Pokémon, along with another legendary Pokémon, Suicune. Celebi takes back a boy named Sammy, who is a young Professor Oak, from the past while trying to flee a bounty hunter. However, Celebi isn't out of trouble yet.

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. 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, you will be able to obtain one.
 * Celebi is the only Pokémon thus far that is able to time travel, other than Dialga, the Time Diety.
 * Celebi has a site that is dedicated to it, as it shares its Japanese name, Serebii.net.
 * Celebi will be in Pokemon Movie 13.