Celebi

According to rumours, on Crystal to get celebi,

You have to defeat all 16 gym leaders and capture all 250 pokemon and the go over to proffesor oak, he will give you a GS Ball. You then give it to Kurt, and after a day he will give it back to you, then you place it by the shrine in the ilex forest and a Lvl30 Celebi will appear!!!

WOW!!!