'M is a name shared by a limited number of glitch Pokémon found in the Generation I Pokémon games. Generally, the name 'M is used to refer to the most common of these glitches, whose internal identifier is
00. This 'M is extremely similar to a specific variant of MISSINGNO., and is mostly harmless. It will, however, freeze the game if a player attempts to withdraw it from a PC box; this has led to misconceptions that it is very dangerous and possibly game-breaking.
'M uses MISSINGNO.'s "fuzz" image as its sprite, and Zapdos' cry as its own. It shares a Pokédex number (000) and moveset (Sky Attack/Water Gun/Water Gun) with MISSINGNO., but not a height or weight; 'M's height and weight are instead 23 feet and 880.6 pounds, respectively. As is the case with some MISSINGNO., catching 'M for the first time will result in the player unlocking 'M's Pokédex data but actually obtaining a Rhydon.
'M appears to be slightly unstable. Chunks of its name are composed of random bytes that do not actually correspond to text characters; this results in pieces of terrain, overworld sprites, and battle sprites appearing on both sides of the "'M" -- in sharp contrast to MISSINGNO., whose name displays normally. Its Pokédex tagline is also glitched, consisting of similarly-random bytes that wrap to the next line (again different from MISSINGNO., whose tagline is simply "???"). Like MISSINGNO., however, 'M has no Pokédex text.
Oddly, catching 'M will not end a battle against it; players who successfully capture 'M will continue to fight against another 'M. Catching this second 'M will earn the player a (perfectly normal) Ditto. (If the player has not yet caught Ditto, its Pokédex data will even be displayed.)
Old Man Trick
Barring the involvement of cheat devices, the species of 'M with hex
00 can always be battled using the Old Man trick.
Though Pokémon Red and Blue will only let players input a seven-character name, room is reserved for eleven characters (including a null terminator, hex
50). As such, there will always be at least three bytes at the end of the player's name whose values are hexadecimal
A wild Pokémon list in Red, Blue, and Yellow holds ten Pokémon, using twenty bytes to do so. This is more than enough to hold the player's eleven-byte name. Furthermore, the first byte of a player's name has no effect on what wild Pokémon appear.
The effect is that unless the player has lengthened their name using a cheat device, any name the player uses will result in at least one entry on the wild list for a level 0 'M (
0000). On a related note, a player with a seven-character name will be able to encounter a level 80 'M (because the eighth and ninth bytes on the wild list would be 5000, and 50 is hex for 80.)
As an example: the name "wwwwwww" is stored as
B6 B6 B6 B6 B6 B6 B6 50 00 00 00. When it is copied to the wild Pokémon list via the Old Man trick, the wild list is stored as
B6B6 B6B6 B6B6 5000 0000 ???? ???? ???? ???? ???? (where
???? denotes bytes that are not set by the player's name and hence are not predictable). The name effectively defines three entries for a level 182 Kabutops Fossil MISSINGNO. (hex B6), one entry for a level 80 'M, and one entry for a level 0 'M.