Glitch Pokémon

Bulbapedia also has an article about Glitch Pokémon.

This article is a summary page for different variations of a glitches, etc. when talked about as a whole.

This article is about glitch Pokémon; the result for when the game attempts to manage values in the species byte for Pokémon, which were never defined by the game developers. For Pokémon with "glitch-like" properties, see glitchy.

The term glitch Pokémon refers to Pokémon that were not intentionally coded into the game. These Pokémon arise as a result of the game trying to interpret some other type of data as a Pokémon and have identifiers that fall outside the normal range for Pokémon. They can be accessed by a glitch or by using a cheating device. Glitch Pokémon in the first two generations use no error trap to give them true sprites and their "images" consist of scrambled data, with the exceptions of some hybrids, and have names that consist of glitch text.

In the third generation, Decamarks all have the "unknown Pokémon" sprite seen in the Pokédex (although it may be corrupted, depending on the index number) and are named ?????????? on the opponents side. Not all glitch Pokémon in Generation III are named "??????????" on the nickname or summary screen. Here, they can have names like shortened versions of move names such as "RBOLT" (instead of "THUNDERBOLT".

Some glitch Pokémon can learn glitch moves. A comprehensive list of glitch Pokémon from the first and second generations can be located on the GlitchDex.