Pokémon 'M, like MissingNo., is found by first going to Viridian City, talking to the old man, and when he asks if you are in a hurry answer "No."
After, fly to Cinnabar Island and surf along the East coast. After some time, you may enter a random battle with 'M. 'M appears as either level 0 or level 80 and it looks like a jumbled mess of pixels. Like MissingNo., 'M is a glitch Pokémon that may cause problems within your version of the game so catch it at your own risk. You have been warned! If you level up 'M at least once it will evolve into a Rhydon also after fighting it or battling any monster that appears on the island edge the item in your 6 item slot in you bag will be able to increase in number. For example: if you have a master ball in the 6th item slot in your bag, you will be able to increase it's quantity by using the left and right arrows while highlighting the item.