MissingNO is a glitch Pokèmon present in the earlier games. MissingNO appears as one of many different forms, and can appear when the player uses certain glitches. MissingNO's summary can cause the game to freeze if it is viewed(or maybe I'm thinking of ----- AKA The Invisible Shiny Bulbasaur, another glitch Pokè). MissingNO can act like other, legit Pokèmon, and can even be sent into battle. However, doing so may cause some interesting things to happen, such as the player automatically losing the battle.
More info on MissingNO: http://bulbapedia.bulbagarden.net/wiki/Missingno.
EDIT: I was wrong, MissingNO is one of the few glitch Pokèmon whose summary does NOT freeze the game.