What is missingno.?

Missingno. (Japanese: けつばん Ketsuban) is a dual-type Bird/Normal Glitch Pokémon. Due to the ease of which it can be found, it is one of the best-known Glitch Pokémon
In later generations, other Glitch Pokémon are referred to as "a Missingno.", despite there being little relation to the one found in Pokémon Red and Blue, except perhaps via a shared number of 000.
Missingno. appears due to an oversight in the programming of the Old Man's tutorial in Viridian City on how to catch Pokémon. When the game sets up the battle between the Old Man and a wild Weedle, it needs to change the player's name (temporarily) to "OLD MAN" so that it will display that name, rather than the player's entered name, during the battle.
Due to the limited space on a Game Boy cartridge, the programmers decided to use the space where data for wild Pokémon found in the grass is stored (which is completely blank in Viridian City; only Surfing and Fishing data is used) to save the player's name temporarily. Normally, this would cause no abnormal activity, as this data is overwritten when the player moves to a different area.
In all cities, however, this data remains blank, and so the data is never overwritten (as there is nothing new to overwrite it with), and thus, the data that was last entered (be it the player's name or the wild Pokémon data from another area) remains in place. This itself still causes no harm; however, an oversight in the programming of the tiles used to denote the shore of an island marks them as equivalent to grass. As all water routes have no real grass on them, likewise, the data is not overwritten, and so whatever data is in the slots for wild Pokémon found in the grass is used, be it the player's name or wild Pokémon found elsewhere, such as the Safari Zone.
The name of the player has six hexadecimal values in it. The game needs only three "slots" of wild Pokémon data to store this.
The species of wild Pokémon the player encounters along the coast are determined by the third, fifth, and seventh characters of the player's name, while their levels are determined by the second, fourth, and sixth characters, respectively. By knowing which letters and symbols correspond to which species and levels, through the use of certain calculators and charts (such as ), it is possible to set the name of the player at the start of the game to find specific Pokémon at specific levels.

Missingno is a glitch pokemon, that first appered in Gold, Silver, and Crystal. It is a glitch pokemon, so it is not actually one. It can not be found in the pokedex. After discovering in in
Gold, Silver, and Crystal, it was taken out of the game for the rest of the games to come. Though, it has appered in anime and in the card game.

MissingNo is a glitch Pokemon (?) that is found in response to watching the old man catch a pokemon then flying to cinnibar and surfing on the coast. Your 6th item will also be duplicated.

yes it is it's like a floating purple ghost.

missingno is a REAL glitch pokemom.
cubones evolution was supposed to evolve to a kanghshan (wrong spelling) in gen 1.
game freak then disalowed it, making them not connected.
the save file was still there and so close to release could not delete it.
Talking to the old man then surfing on the coast of cinnabar island made the evolution file (or missingno as your asking) attack as a wild pokemon.EASY PEASY