The GS ball in which Celebi was caught was sealed tighter than a Master Ball, closed so shut that professors failed to open it and a man who made pokeballs from apricorns had to be called in. And yet when he opened it, with cameras surrounding him, it was empty. At first, speculation centered on foul play – had the delivery man or some professor or the apricorn man himself opened the ball? Soon, however, forensic tests revealed that the ball had never been opened since Celebi was captured!
There are no pokemon capable of teleporting from inside a pokeball; if there were, Abra could never be captured for long. There was no explanation short of time travel, and although legends discussed it, physics held it to be impossible. The research team who had captured them was humiliated by this escape, denounced as frauds, their reputations ruined. They were even more shocked a few weeks later, when they discovered that the evidence that they had ever captured it, their public shame, and everyone else's memories of the GS ball had simply disappeared!
There was only one explanation for this, no matter how unlikely it seemed. Celebi, ever free-spirited, had gone back in time to prevent its own capture. So were Gold and Silver stymied, so have generations of greats failed. The saga of the disappearing Celebi has been repeated through the ages, forever and yet never through the magic of time travel.
This concludes the Pokedex entries for the Johto region. I'd like to thank everyone who's helped me out along the way, be it ideas, inspiration, or pointing out errors. See you in Hoenn!