Glitch Pokémon (JP: "Error Pokémon") are the mysterious Pokémon seen as part of The Road to XD^3, in The Curse of the Terrifying Glitch Pokémon.
Glitch Pokémon do not have a set type. They are said to be of the "Bird/Normal" type, but this does not exist. Therefore, they are generally recognized as simply "glitch type," with no weaknesses or resistances.
There are two main Glitch Pokémon, Missingno and the weaker 'M'. Subsets include glitches in the form of skeletal ancient Pokémon or a ghost (see below.)
Both Missingno and 'M' terrorized Cinnabar Island in The Curse of the Terrifying Glitch Pokémon.
M (also: M-Block, Chip-M-Block) is the weaker of the two Glitch Pokémon. It does not possess many remarkable characteristics besides having the ability to fire Water Gun twice at a time and use Sky Attack. Even though it looks like Missingno, it is much weaker and does not possess any of Missingno's special powers. It was easily destroyed by Rich and Anabel when they simply flushed out its weak point then struck it.
Missingno (Full: "Missing Number," also "Missingno.") is the fully powered, ultimate Glitch Pokémon. Crafted for evil deeds by a wicked genius, Missingno sometimes appeared in 'M's place on the eastern beach of Cinnabar Island. It is hundreds of times more powerful than its brother, able to easily crush even the strongest Pokémon.
However, Missingno's truly frightening characteristics are the special ones its creator gave it. When it defeats a Pokémon, it can drain that Pokémon's soul as nourishment. Not only that, the drained Pokémon slowly transforms into a Glitch Pokémon itself. Missingno also possessed the power to transform into the aforementioned skeletal Kabutops and Aerodactyl along with a ghost. Its trademark attack is Glitch Void, which sucks its target into Missingno's body and sends it to "Glitch City," a dimension tucked within Missingno's being.
'M' and Missingno were created as part of a cruel experiment by Professor Willow. He had schemed to conquer the world utilizing Missingno's power to drain the souls of Pokémon. To create his monstrosities, Willow forcibly stole the data contained within a Porygon - seriously hurting it in the process - and corrupting it into the forms of 'M' and Missingno. He controlled every move 'M' and Missingno made from a secret laboratory in the basement of the Cinnabar Mansion, including sending Rosemary, then his assistant, to Glitch City when she almost discovered his secrets.
He nearly got away with his plan, too, when he sent Anabel to Glitch City following an unsuccessful attempt to defeat Missingno. However, Rich struck out with only Anabel's Gallade by his side and, together with Cinnabar Gym Leader Blaine, discovered Willow's secret lab after rescuing the injured Porygon. After revealing his true identity as the mastermind of the Glitch Pokémon, Willow battled Rich head-to-head. Gallade was no match for Missingno, so Rich had no choice but to send in Porygon. After using Lock-On, Willow ordered Missingno to consume the Pokémon with Glitch Void, but Rich sacrificed himself and stepped into the attack's path, meaning that he was sent to Glitch City instead. This bought Porygon the time it needed, and its Zap Cannon blew Missingno apart (Missingno was made from Porygon's data, therefore Porygon was the only thing that could defeat it.) Upon its destruction, all the souls were restored to the Pokémon Missingno defeated, and Rich, Anabel and Rosemary were freed from Glitch City and restored to life. Willow made a last-ditch effort to stop them by blowing up his lab, but Blaine's Tangrowth helped pull him from the building and into the custody of the police.