It is found in Pokémon Yellow by doing Method #3 of the Mew glitch with a Pokémon with a special stat of 195. It can also be obtained by doing the Johto guard glitch with Quilava. Its Red/Blue equivalent is h POKé. However, since h POKé's Pokédex entry freezes the game, some players might want to trade a ゥ ( Z4 in order to get h POKé.
This glitch Pokémon article is part of Project GlitchDex, a Bulbapedia project that aims to write comprehensive articles on glitches in the Pokémon games.