It is found in Pokémon Yellow only - it can only be found by doing Method #3 of the Mew glitch with Pokémon with a special stat of 196. It can also be obtained if a PokéWTrainer is traded from Red and Blue.
As it learns the HM technique as a starting move, and cannot be traded to any game containing a move deleter, X ゥ- xゥ, is unable to forget Flash. However, if obtained at Level 12 or higher, or traded from Red/Blue, it will not know Flash.
This glitch Pokémon article is part of Project GlitchDex, a Bulbapedia project that aims to write comprehensive articles on glitches in the Pokémon games.