MissingNo

MissingNo is a Glitch Pokemon in Generation 1. It has the types Bird/Normal in Red and Blue and the types Normal/999 in Yellow. MissingNo is the most well known Glitch Pokemon. Normal Blue/Red Form MissingNo shares a sprite with 'M (00)

MissingNo has 5 forms and 39 different Pokedexjkjdlfjdals

Numbers. The Normal Yellow form for MissingNo is NOT safe to encounter as it normally causes the game to freeze or become stuck.

Normal Red/Blue Form
Old Man Glitch, Mew Glitch and the Time Capsule Exploit

Normal Yellow Form
Mew Glitch and Time Capsule Exploit

Fossil MissingNo
Mew Glitch, Old Man Glitch and the Time Capsule Exploit.

Ghost MissingNo
mew Glitch, Old Man Glitch, Time Capsule Exploit and evolving  .PKMN(Red/Blue) or Z ゥ (Yellow) at level 224

Trivia

 * 9 of the 39 MissingNo have cries that are not 00 values. This supports some of the speculation that MissingNo were actually removed Pokemon. This is supported further by Shigeki Morimoto, a pokemon designer, who has said that there were originally meant to be 190 Pokemon.
 * MissingNo. is referred to as "Pokémon 000" on Nintendo's consumer support website, referencing its Pokédex number.
 * MissingNo is the only Glitch Pokemon to have an intentional depiction from a franchise artist.