Navel Rock Ho-Oh .

Using the Mystic Ticket received from events on their copy of Pokemon FireRed & LeafGreen, or Pokemon Emerald games,
players gain access to the Navel Rock, and get a chance to capture to Legendary Ho-oh!

For players who rather not use the Mystery Gift to activate the event, simply use PKHeX's Event Flags > Research, and
For Pokémon FireRed & LeafGreen: Check Flag 2122 and 2123
For Pokémon Emerald: Check Flag 0315 and 2272
then proceed to add a Mystic Ticket into the Key Items of the Bag.

The shiny Ho-oh here is obtained via Tool Assisted RNG.