Both Missing No and M are glitches, from Gen 1-Gen 2. They were a mistake caused by havingthe Pokeball Tutorial able to be done multiple times (which stopped in Ruby/Sapphire). By going to the guy who teaches you to use the Pokeball in R/B/Y, then Flying to Cinnabar Island and Surfing directly along the East edge, you meet Pokemon above level 100 that shouldn't be there. Then, you will meet Missing No. Missing No can have different names, appearnces, and HP bar length (one went all the way across the screen) plus knows these moves: Sky Attack, Water Gun, another Water Gun, StinkerstubbleU. StinkerstubbleU is actually able to kill your Pokemon (not faint kill.) If you defeat Missing No, the sixth from the top item in your bag is increased x128 (it clones your items, most often used on Master Balls and Rare Candies.) If you capture it, then it is likely that your game will restart. However, this is not always the case. Sometimes the game just gets incredibly glitchy. Missing No is also likely to make an appearance during a Link Battle. Now onto M. M is Missing No's meaner brother. M sometimes appears instead of Missing No and automatically starts over your game. It is very rare however.