'M (00), The Glitch You Probably Caught Instead of MissingNo.
I have some news about your childhood, and I need you to sit down for it. That backward L you caught off the coast of Cinnabar Island, the one you have been calling MissingNo. for twenty years? There is a real chance it was not MissingNo. at all. It might have been 'M, the identical twin nobody talks about, and the two are different Pokemon with different rules. Let me introduce you properly, because this one has a few tricks its famous sister does not.
Same face, different soul
'M sits at hexadecimal index 00, the very first slot in the game's list, and it wears the exact same backward-L sprite and the same Pokedex number 000 as MissingNo. Bulbapedia calls it the sister glitch, and on screen you genuinely cannot tell them apart. The differences live in the data. Look at the stats side by side and you will find they are almost, but not quite, the same creature:
Base stats ('M, documented)
Attack 137 against MissingNo.'s 136. One point. 'M literally out-punches the most famous glitch in history by a single point of Attack, and then hands back the advantage by having a Speed of 6 where MissingNo. has 29. Both share the perfect zero in Defense, so neither one is surviving anything with muscle behind it. A base stat total of 205 versus 204. Two spirits fighting over the same paperwork.
The party trick MissingNo. wishes it had
Here is the thing that makes 'M special. It evolves. A corrupted slab of memory with Pokedex number 000 can level up and turn into a completely normal Kangaskhan, and under other conditions into a Clefairy. MissingNo. cannot do this at all. I want you to picture that moment: the game is holding a pile of garbage data, shrugs, and produces a healthy Kangaskhan mother with a baby in her pouch. Generation 1 was held together with hope, and occasionally the hope paid out.
This was also the practical reason people caught 'M on purpose back in the day. Kangaskhan is a one-per-game encounter in the Safari Zone, and 'M was a renewable source. A glitch Pokemon with a legitimate use case, that is rare company.
Where the twin bites
The encounter rules have their own quirks. 'M comes from the old man glitch like its sister, but it cannot be reached through the Mew glitch at all, and which of the twins you actually met back in 2000 came down to the characters in your name. That is the genuinely strange part: whether your childhood glitch was MissingNo. or 'M was decided the moment you typed your name on the title screen.
It gets weirder at the border
Trade 'M to Pokemon Yellow and it stops being 'M. It arrives as a different glitch Pokemon called 3TrainerPoke $, which is what happens when Yellow reads Red and Blue's garbage with its own accent. And it can never reach Generation 2. The trade machine shows it as a Slowpoke or a Tyrogue in the preview, but the trade will not complete honestly. Like every glitch Pokemon, 'M is sealed inside the generation that accidentally created it.
The verdict
'M is the better story wearing the same costume. One point stronger than its famous sister, six times slower, evolves into a kangaroo, freezes your game from inside a filing cabinet, and changes species when it crosses a border. If MissingNo. is the landmark, 'M is the local legend the landmark gets credit for. Next time someone tells you about the MissingNo. they caught as a kid, ask them what their trainer name was. There is a decent chance they have been telling someone else's story this whole time.
Sources: documented figures and behavior from Bulbapedia's 'M (00) research. Companion profile: MissingNo., the complete profile. More specimens live in The Glitchhouse.
Go deeper: 'M in the Glitchhouse · battle it in the engine.