Missingno was a potentially game-breaking bug that, nonetheless, thousands of kids risked their Pokémon Red and Blue saves to uncover. Despite the fact that the method for doing so was a total ball ache and not guaranteed to work. The name, is because the Pokémon is literally a missing number, a bit of code that wasn't meant to be used, likely a test creature for debugging. That's the actual explanation for Missingno is, anyway, but some sadist obviously had to cook up an in-universe background for the character. And it isn't nice. Which of these fan theories is? People sure do like messing with their childhood memories. And this one potentially screws with two. Basically, it boils down to the idea that Missingnos are Pokémon who have spent too long in their Pokeballs. It's suggested they're transferred into digitial data when dormant in your belt or the computer and then their data gets corrupted. Yeesh. Double theory, though: perhaps Digimon are also damaged Pokémon...?