MISSINGNO is a Pokémon that appears in the Red/Blue Pokémon GameBoy game only.
Its number is #0, and it's extremely weak defence-wise, type Flying/Water, and starts from level 80.
MISSINGNO is, as you may have guessed, a bug. It has no character graphics (appears as random jumble), and even seeing it in the game may mess up the Hall of Fame list.
You may find it by surfing along the beaches of Cinnabar Island - but remember, you've been warned!
To play along with the joke, Pokémon Gold/Silver has one "official" new Pokémon that looks extremely weird and changes appearance... =)
See: PokeDex