From Bulbapedia, the community-driven Pokémon encyclopedia.
X - x (literally X ゥ-x ゥ,)is a glitch Pokémon found in the Yellow version 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, which is X - x's red/blue equivalent, is traded from the Red/Blue versions to Yellow).
At the GameBoy Tower in Pokémon Stadium, when X - x appears, the screen where X - x is will appear purple. X - x's number is #203 and its starting moves are Skull Bash, Flash, Constrict and Waterfall. Its types are Blank and Normal.