Ilex Forest Celebi .

In Pokémon Crystal, lies an event encounter whereby users get to challenge (and potentially capture) the Mythical Celebi.
Problem is, the encounter required the usage of the Mobile Communication Device, that was scrapped from the International Versions of the game.
As such, this event became unused for a good 10 odd years. (Using the cheat code 91C089DA, players can immediately get the GS BALL from Kurt, and proceed to get the encounter.)
 
As of 26th January 2018 12AM (Australia time), Pokémon Crystal (international versions) has been released to the Virtual Console,
with the only requirement for obtaining it being defeating the Elite Four. (Any instance would unlock the flag. Any other step, such as bringing the GS Ball to Kurt and waiting for one day still applies.)
This allowed Celebi to be legitimately captured and transferred up to generation 7.

This file was first uploaded to our Unreleased Gallery, as the Celebi event was unused at that point in time.
This page hosts the Celebi, obtained from the encounter.

The details below reflect the encounter, not the file itself.

 

Species		Celebi
Nickname		(default, in )
OT		(recipient)
TID		(recipient)
Distribution		In-Game Encounter
Location		the Johto region
Dates		(Various)
PID		Chance of Shiny PID
Games		VC Crystal

	Lv. 30

Nature		Random
Ability		Natural Cure (1)
Item		None

Moves   (ITALIC means Relearnable)   Heal Bell   Safeguard   Ancient Power   Future Sight

Format Ver.1.0.6-0, Post Updated Date:20180126_0156

(Inside the Spoiler, shows GIFs detailing the encounter)

It's possible to use some glitches to obtain it, as seen here, from Chickasaurus' video: