Ilex Forest's Celebi .

In Pokémon Crystal, lies an event encounter whereby users get to challenge, and potentially capture, the Mythical Celebi.
In the Japanese versions of Pokemon Crystal, this encounter required the usage of the Mobile Communication Device.

As of 26th January 2018 12AM (Japan time), Pokémon Crystal has been released to the Virtual Console of the Nintendo 3DS, 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 page was previously listed as "Classic", but is now reflected as "In-Game One-Off",
signifying that it can be encountered in the Virtual Console release without any additional glitch, hacking or modification of any sorts.

The details below refer to the encounter, not to 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