I doubt anyone is going to believe this, but during my first playthrough of Pokémon: Black Version, the Reshiram I got upon playing the game was shiny. There's only one chance to get this Legendary in the entire game (I'm pretty sure it cannot be bred), so that is insanely lucky. I still have the game, though I am not sure where the card that is inserted into the 3DS is.

Description: https://pixelartmaker.fandom.com/wiki/User:MrTroublemeyer