"Missingno.", presumably for "missing number", is not so much a Pokemon as the result of a programming oversight in the Game Boy Pokemon games RED and BLUE. Missingno is best known as the Pokemon responsible for:
- Screwing up your Pokemon cartridge if you capture it;
- Allowing the (in)famous "Rare Candy Cheat";
- Crashing your Hall of Fame data.
There is also a Pokemon similar to Missingno. with a glitched name consisting of block shapes and the letter 'M'.
So what's the mystique? Basically, Missingno. has acquired so much notoriety due to his presence in many of the Pokemon myths, especially involving Mew, the much-vaunted and non-present #151. While the myths aren't true, Mew's popularity ensures that Missingno. will always have a place in Pokemon culture.