'M (hex FF), known more commonly as Charizard 'M by fans, due to being a hybrid of Charizard, is a glitch Pokemon in Red and Blue. While it has the same name, it's completely unrelated to 'M (hex 00) or 'M (hex FE).
Charizard 'M can only be obtained by trading a Q to Red and Blue, which will cause it to turn into a Charizard 'M.
Like its Yellow counterpart, Q, it hides everything below it. In order to heal Pokemon at Pokemon Centers below a Charizard 'M, it must be switched to the last party slot.
Also, if a Pokemon above Charizard 'M is deposited in the PC, it will create unstable hybrid Pokemon and multiply. This can be avoided by switching Charizard 'M to the last party slot.
Like Q, Charizard 'M needs its own box because it also makes hybrids when a Pokemon is withdrawn with something below it. This can cause total destruction of a PC box. If all the player's Charizard 'M are in one box and not deposited into another box, the player will be safe.
Charizard 'M is a hybrid of Charizard, and shares its sprite and typing with Charizard (Fire/Flying).