MissingNo. (abbreviated from Missing Number) is a Bird/Normal-type (Normal/999 in Pokémon Yellow) Pokémon from the Pokémon series. However, it isn't an actual Pokémon, but an error handler (or placeholder) for when a nonexistent Pokémon is encountered. 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 depending on what the player's name is, and four being exclusive to Pokémon Yellow. It appears at #000 in the National Pokédex, though it has no Pokédex entry in-game.
MissingNo., being a glitch, has the potential to mess with save data (in this case, the Pokémon Red and Blue save data) by doing certain things like duplicating the item in the sixth slot of the bag by 128, ruining the Hall of Fame data, and scrambling up the game's graphics. However, other then the Hall of Fame data corruption, the glitches can be fixed. Looking at a normal Pokémon's stats will fix the graphics errors in battle. And the items bug (if you want to), you can toss out the items. However, do not have a key item in the sixth slot, or it will be duplicated, thus preventing you from storing it in the PC.
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.
An edit of Mike's Missingno., this MissingNo. has been given a complete visual makeover, giving it sprites from the original Pokémon games, as well as a new portrait, icon and cry. It also includes the other sprites of MissingNo., including the Kabutops fossil and the Aerodactyl fossil). The attacks, however, remain unchanged. The sprites have been recently updated by said person along with win quotes being added.
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 Shun Goku Satsu 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.