There are many, many glitch Pokémon, so it would be easiest to read for yourself here. The majority of them only exist in the first few generations, although they have been discoverd up to Gen IV or V. Here are a few of the coolest / most famous glitch Pokémon.
By far the most infamous glitch Pokémon. Found only in RBY, it is unknown what it's supposed to be. There are theories that it was a discarded Pokémon, or that it's a failed attempt at generating an regular Pokémon. It has five distinct forms. The most common is the one in the upper left corner (Red and Blue), followed by the yellow one in the bottom right (Yellow). On rarer occasions it will apear as the Pokémon Tower ghost, or an Aerodactyl or Kabutops skeleton from Pewter Museum. In Pokémon Red and Blue Missingno. Is Normal/Bird type. In Yellow it is Normal/Glitch type. Missingo. has never appeared in later games, although several other glitches have been called "Missingno."
While it looks like Missingno. at first glance, 'M (00) differs in several ways. It can evolve into Kangaskhan (leading to a theory that Marowak was supposed to evolve into Kangaskhan) and cannot be caught via the Mew glitch. It's typing is Normal/Bird and is only found in Red and Blue.
Otherwise known as "-----" and "Invisible Shiny Bulbasaur", this glitch is found in DPPt and is a Normal-type Pokémon with the cry of a Bulbasaur. When sent into battle it shows the Shiny animation. However, it cannot be seen and appears as a box of green static.
Decamarks is found in the Gen III games (RSE and FRLG). While technically not a glitch Pokémon because it was programmed into the game as a sort of "anti-glitch", it can still be encountered, caught and will exibit glitchy behaviour.