Glitch Pokémon

Bulbapedia also has an article about Glitch Pokémon.

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 do not have 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 and are named ??????????. Most Glitch Pokémon can learn glitch moves. A comprehensive list of all Glitch Pokémon are located on the GlitchDex.