MissingNo. (abbreviated from Missing Number) is a Bird/Normal-type (Normal/999-type in Pokémon Yellow: Special Pikachu Edition) glitch Pokémon from the Pokémon series. It is a glitch Pokémon that, although not usually being able to evolve (sometimes evolving to a random level 1 Pokémon), has eight different forms that it can appear as, with four of them dependent on the player's in-game name, and four being exclusive to Pokémon Yellow: Special Pikachu Edition. It appears at #000 in the National Pokédex, though it has no Pokédex entry in-game.
Although MissingNo.'s appearance can differ depending on the game and what method was used to encounter it, its most recognisable form is the L-shaped entity from Pokémon Red and Blue, comprised solely of corrupted graphics. MissingNo. is not an actual Pokémon, but is instead an error handler used by the game for when data for a non-existent Pokémon is encountered. MissingNo. was first implemented in the Japanese Pocket Monsters Blue as a side effect of correcting a bug present in Pocket Monsters Red and Green that caused these two games to crash. Because the international releases of Pokémon Red and Blue are both built using Pocket Monsters Blue as their framework, MissingNo.'s existence was carried over into these games. In Pokémon Red and Blue, MissingNo. has the potential to mess with the save data by performing certain actions like corrupting the Hall of Fame data and scrambling up the game's graphics, though most of these glitches can be fixed without deleting the save data and starting a new game. However, MissingNo. became the most well known glitch due to its effect of duplicating the item in the sixth slot of the player's inventory upon being encountered, which can easily be exploited to the player's advantage.
In M.U.G.E.N, MissingNo. has been made by Mike, KoopaKingdom.com and The_None. Mike's and The-None's versions are both a scratch-sprited, though only Mike's attacks using moves that it learns in the Pokémon games, such as Water Gun and Sky Attack, as The_None's has a moveset akin to a lot of his other characters'. KoopaKingdom.com's version is a spriteswap of Human Smoke from Mortal Kombat.
MissingNo. is a six-button character that was made cheap deliberately, due to it being a glitch and not a real Pokémon. It has the ability to spam most of its moves, leaving the opponent in an endless loop until they're K.O.'d. One of its attacks, Crash, has the potential to O.H.K.O its opponent(s), unless they block, or have a high Defence stat.
The_None's characters are known for their excessive randomness and strange attacks, and MissingNo. is no exception. Although since MissingNo. is a glitch, this makes it slightly more reasonable. It can transform its body randomly unlike the others, allowing it to make various references to other media. The hitboxes of its attacks have very large range, making it somewhat hard to take down, but can be done with some skill. It can also flash the infamous faces from Game Boy Camera at random during some attacks. In addition, the Raging Demon of this character can make the opponent glitch out with random effects for each character, some not being able to perform combos and others not being able to move. Visually, this character doesn't look a whole lot like MissingNo. at all (minus the L-shape), though one of its palettes matches the colours of the real MissingNo.
MUGEN Missingno. vs. Setsuna Sakurazaki
TK's Random Mugen Battle 914 - Buttercup VS MissingNo
Mct Mugen-Missingno. vs. Blaziken
Mugen Rajaa's Warachia (Me) VS Missingno. by The None (AI)
TBM Mugen Match 100 - Mugen Special Missingno. Beatdown