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This article contains fan speculation. There is no solid evidence for or against some parts of this article.
Over the course of the Pokémon series, a number of creatures with traits of Pokémon have appeared. Their status as Pokémon is often disputed.
List of disputed Pokémon
In the games
Egg and Chick
In Pokémon Red and Green there is unused text for extra badges, "Egg" and "Chick", "Bronze" "Silver" and "Gold", and what appears to be the names of unused Trainer classes. These strings are defined in the ROM directly after floor names, which follow on from item names, meaning that the unused text can be forced as glitch items. The text remains untranslated in the localised Red and Blue, and Egg and Chick are kept with the names "たまご" and "ひよこ" respectively. This results in illegible text in the English versions, because the character table for the localised Generation I games is incompatible with Red and Green. It is unknown what "Egg" and "Chick" would have referred to, or whether the concept was a precursor to Pokémon Eggs. The text matches early Capsule Monsters artwork in which an unknown creature can be seen fleeing from an egg. The concept may also be reminiscent of the Fresh egg and Chick items in EarthBound, in which the player would keep the Egg until it hatched into a Chick, and then raise the Chicken and eventually sell it. EarthBound was a game that Ape Inc. developed; the company that eventually became Creatures, Inc.
While the Substitute doll originally looked similar to Rhydon, the design evolved over time to a more neutral design. In Super Smash Bros. for Nintendo 3DS/Wii U, its trophy description points out that, while it looks and gets attacked like a Pokémon, it is not actually one. The doll has gone on to receive several pieces of merchandise, including its own plush doll and clothes.
In the anime, Shellder is required to evolve Slowpoke into Slowbro or Slowking. Shellder clamps onto a Slowpoke's tail to evolve into Slowbro, or onto its head while wearing the King's Rock to evolve into a Slowking. Even though Shellder changes form drastically when it bonds with Slowpoke to evolve, it is not considered an evolution in and of itself. Mega Slowbro has the Shellder engulf most of Slowbro's body; essentially making it a shell for Mega Slowbro.
Kangaskhan carries around a little blue baby Kangaskhan in their pouch. The baby Kangaskhan lacks the ridges and patches on its mother's body, and its small fangs are not visible outside of its mouth. Even with the advent of breeding and baby Pokémon, the baby Kangaskhan has yet to be made into a separate Pokémon, and as a result, all Kangaskhan are born with a baby already in its pouch. As part of Kangaskhan's Mega Evolution, however, the baby is capable of fighting alongside its mother.
It is said that the Legendary beasts Raikou, Entei, and Suicune were reincarnated after they died in the fire that burned down the Brass Tower by Ho-Oh. They were said to represent the lightning that struck the tower, the fire that burned the tower down, and the rain that put it out. While it is unknown in the games if these three were already a Raikou, Entei, and Suicune before they were revived, the Pokémon Generations short The Reawakening showed that they were three, nameless Pokémon.
According to interviews with Ken Sugimori in Nintendo Dream, the bones that Vullaby and Mandibuzz wear are those of other Pokémon. However, it is not stated which Pokémon they are from, and as the skeletal systems for the majority of Pokémon have yet to be revealed, it is unknown if they are meant to be the bones of a specific Pokémon. However, the skull that Vullaby wears as a diaper is similar to a human skull, which may indicate that the bones are from a Pokémon in the Human-Like Egg Group.
The origins of Zekrom and Reshiram are told in legends, as they were once a single powerful Dragon Pokémon used by twin heroes in order to create the Unova region. But the brothers each sought something different in life—truth for the older brother and ideals for the younger—and they began to argue, then fight, over whose side was right. The single dragon, in response, split into two Pokémon: Reshiram, who sided with the older twin, and Zekrom, who sided with the younger twin. According to Ghetsis in Black 2 and White 2, Kyurem is what is left of the original dragon Pokémon.
While Genesect is a cyborg Pokémon, it did not previously take this form. The modern Genesect was revived by Team Plasma from a fossil of this Pokémon, and modified into its present appearance. According to Genesect's Pokédex entries, it existed over 300 million years ago, and was feared as a powerful hunter. Due to its lack of Team Plasma alterations, it is unlikely that it would have been part Steel-type, nor would it have access to Techno Blast or similar moves.
In Genesect and the Legend Awakened and its manga adaptation, fossilized parts of the original Genesect's body are seen. These fossils bear a great similarity to the bodily structure of the modern Genesect. However, these fossils have not yet been seen anywhere outside of the anime.
A silhouette possibly of a Pokémon was discovered while rummaging through coding of the recently launched Trading Card Game Online. This silhouette was discovered in the navigation bar on the game's tutorial page, combined with images of the Unova starter Pokémon. No additional information on the silhouette was contained in the source code of the tutorial's Flash file. According to Serena Robar, a community manager for The Pokémon Company International, it is not a new Pokémon, but is actually just a placeholder image that wasn't removed after the code was decompiled.
Bittercold is a living lifeform that is trying to destroy Mist Continent. It can be fought in Glacier Palace's Great Spire. Although Bittercold is not considered to be a real Pokémon, it used unnamed moves to hinder Pokémon.
Dark Matter is a living lifeform that is trying to send all Pokémon to the Voidlands and to return the Pokémon world to the void. It can be fought after clearing Tree of Life: Trunk. Although Dark Matter is not considered to be a real Pokémon, it can use moves against Pokémon and can give itself a random type.
The only known information about this creature is from its brief appearance in The Ancient Puzzle of Pokémopolis. It seems to be a bear with decorative markings covering its body. Since markings like this are also present on the other Pokémopolis Pokémon featured in the episode like Gengar and Alakazam, it is unlikely to have been a normal feature.
This creature made its sole appearance in The Superhero Secret. It seems to be a giant yellow spider, with four red compound eyes, two antennae on its head, and a mouth with four large fangs. Its thorax has eight long, brown legs. Its abdomen is much larger relative to the rest of it body, with two thin, black stripes. Its cry is a screech, not unlike Victreebel's. The nature of its appearance (in a Gligarman movie) would appear to indicate it may be a fictional monster or Pokémon, and whether it is supposed to be portrayed by a real Pokémon, a mecha, or simply special effects is unclear, though its movements and cry give it an extremely organic appearance. Due to its rather early appearance in the Johto series of the anime, it is unlikely to have been intended to be in the games, and can be considered to be an anime-exclusive creature.
In a sequence where the main characters see their dreams for the future, Misty sees herself chasing an unknown "legendary water Pokémon". The only part of the Pokémon the viewer sees is a giant fin, which is reminiscent to that of the then-unreleased MegaGyarados.
Unreleased Unown seen on Professor Oak's computer. Due to the introduction of two additional Unown forms in Generation III, some fans have speculated that these may have been very early prototypes for new forms of Unown. Some of the Unown resemble various Greek and Cyrillic letters, namely Я, Ω, Σ, б, Д, ω, Π, and σ.
An evil monster that attacked the city of Alto Mare was seen in the Japanese version of Pokémon Heroes: Latios & Latias. The storybook-style segment of the movie, which told the story of the origin of the Soul Dew and Alto Mare's ancient history, was completely cut from the dub.
Marin (Japanese: マリンMarin) is a Pokémon appearing in the Magical Pokémon Journey manga. It appears only on the title page of The Best Gift Ever. According to commentary by Yumi Tsukirino, Marin was designed by a fan in a "draw Pikachu's friend" contest in Japan held by Ciao magazine (Japanese: ちゃお) and first published in the December 1998 issue of the same magazine in which the cover page is presented in color, and Marin's body is shown to be entirely blue.
After seeing the two Pokémon for the first time in Spear Pillar, Pearl theorized that the statue could represent how ancient people saw the two Pokémon fighting inside a distortion of space and time which led them to believe it was one Pokémon.
Brock is shown with a large Pokémon collection at the Pewter Gym. Many of them are not identifiable as currently known Pokémon. One of them on the top left shelf resembles the unnamed Bug-type Pokémon encountered by Red earlier in the manga.
MechaMew2 is a Robotic Pokémon seen only in Pokémon Live!. It is considered by its creator Giovanni to be the most powerful Pokémon in existence. Giovanni promised Trainers the very rare Diamond Badge to anyone who could defeat it. MechaMew2 is based on Mewtwo, another Pokémon whose creation was ordered by Giovanni. It destroys itself at the end of the show after learning an attack of love, caring, and friendship.