'M is a glitch Pokemon found in generation 1 pokemon games. It can be very dangerous to your game if you catch it. However, it is only as dangerous as seeing a Missingno. (and just as useful, because the same Rare Candy Cheat goes through) if you just see it. When found, it will be at either level 0 or 80.
Trivia
Most people call the ghost form "Missing No", and the random pixel form "M".