Kotora

An unused design that ended up being cut for 2 Generations.

This design was first seen by players in the Gold/Silver '97 Spaceworld Demo.

It was later discovered to also be cut from the prototype versions of Generation 1, and never made it into the final games.

Its Japanese name can be seen as small tiger or tiger cub.

Mitei 12

Appears to be a prototype Celebi.

This design was first seen by players in the Gold/Silver '99 Spaceworld Demo.
Though I believe they did not get to see it in the demo itself, but rather via the leaked game.

The name basically means 12th placeholder.

Okutank

It has the same Japanese name as Octillery, so it is almost evident that this is a beta Octillery.

This design was found in the data for the Gold/Silver Spaceworld '97 Demo.

Its Japanese name could be a reference to Octopus and Tank.
The tank-like elements are more apparent in this design than that final design.

Poponeko

Based on various similarities between its family in the demo and in the final games, it appears to a beta Skiploom.

This design was found in the data for the Gold/Silver Spaceworld '97 Demo.

Its Japanese name appears to be a reference to dandelion (popo from tanpopo) and cat (neko).
The cat-like elements are more apparent in this design than that of the final design.

Presumably the cat-like element was cut for the final games, hence why it looks less like a cat in the final build,
and it was subsequently renamed in the final build.

Marill

Evidently a beta Marill.

This design was found in the data for the Gold/Silver Spaceworld '97 Demo.

The only thing notable about it, besides it being a rough look different from the final design,
is that it apparently has no evolution at that point in development.

Pii

Evidently a beta Cleffa.

This design was found in the data for the Gold/Silver Spaceworld '97 Demo.

The only thing notable about it, besides it being a rough look different from the final design,
is that it evolves at level 12, instead of being evolution via happiness.

Nyorotono

Evidently a beta Politoed.

This design was found in the data for the Gold/Silver Spaceworld '97 Demo.

Akueria

This design was found in the data for the Gold/Silver Spaceworld '97 Demo.
It is meant to be the final evolution of the water starter received in the demo. It shall be noted that Totodile is not the water starter for this demo.

Taaban

This design was found in the data for the Gold/Silver Spaceworld '97 Demo.
It looks like it was meant to be the Shellder that got detached from a Slowbro/Slowking.

Despite its apparently likeness, in the game's data, it has no evolutionary relationship to both the Slowpoke and Shellder lines.

Bangirasu

Apparently a beta Tyranitar.

First seen on the cover of the April 1997 issue of MicroGroup Game Review. The Spaceworld '99 sprite matches the final build.
The primary differences would be the change of the sprite's color, as well as the cubital fossa area being indented. This can actually be observed on the final sprite for Pokémon Gold, if that area was colored similarly to the belly area.

Lizardon

The beta of a female Charizard.

First seen (in terms of game data) in one of the earliest prototype builds of Diamond and Pearl. It appears to be an attempt to add several early official artwork that depicts Charizard having only a single horn to the official canon.

It only has one horn instead of two horns. This gender difference was never implemented into the final builds of Diamond and Pearl. There is no clue why this gender difference was avoided.

Aruseus

A beta version of Arceus.

First seen from the June 30th Prototype Build of Diamond and Pearl. This version of Arceus already has Multitype as an ability.

The default color of grey changes when it becomes other types.
 

Karanakushi

Reimagining based on leftover back sprite of Shellos from the final builds of Diamond and Pearl.

Looking at the prototype build, this ambiguous back sprite was used, while it already uses the standard West/East sprite for the front sprites. The back sprite's color changes depending on the form. The same phenomenon was observed for Gastrodon.

At this stage, it is not certain whether a single unified Shellos & Gastrodon existed before they were split to West/East variation, or this back sprite was simply meant to be used for testing purposes.

Nicknamed by fans Latiken

A prototype Pokémon that likely spawned the designs of Latias and Blaziken.

This design was first seen in a pre-alpha sketch. After the pre-alpha sketch, official artwork was released that appears to match a similar style of the pre-alpha sketch.

F-00

Unused shiny coloration for the Pokestar Studio opponent.

First seen in Pokémon Black 2 and White 2's data.

Eternal Flower Floette

A 6th form of Floette, never released to players.

This Floette was first seen as a Pokémon owned by AZ in the games Pokémon X&Y.

This Floette cannot evolve, and be evolved into. It has 1 less base stat than the base stat total of Florges.
It also has a signature Light of Ruin, that cannot be learned by any other Pokémon (besides Smeargle using Sketch on it).

As of time of writing, this Pokémon was never released to players, despite the full data being in XY, ORAS, SM, USUM.
The personalinfo is technically in LGPE, despite the model not being programmed into those games.

Gogoat

An unused shiny form of Gogoat.

This unused shiny variant was first shown in an official Pokémon X&Y pre-release footage. Its shiny variation was changed upon the release of the final build.

It is important to note that the non-shiny variant of Gogoat has been revealed prior to this, and in other parts of the video itself one can see regular Gogoat. Hence why this variant is believed to be the shiny variant, and not a recoloring of the regular variant.

Ash Greninja

An unused shiny variant of Ash Greninja.

At this point in time, Ash Greninja was never made available as being shiny to players.

Ghost

Ghost of the Mother Marowak. The model is cool, but not obtainable by the player.

A story element from the Generation 1 games that gained a 3D model. Like the Generation 1 games, this creature was not meant to be owned by players.

Eternatus

An Eternatus model from the prototype builds of Sword & Shield.

This may not have been an intended design of Eternatus, but rather a bare-boned model for testing the script and story functions of the game.

Beta Cyndaquil (Probably)

Info I have about this is a lil' sketch. If I'm not mistaken, it was found within a leak containing various GS betas.
It wasn't found in one of the games, but instead within a backup of sprites found within the source code for Korean localization process.
Additionally, a palette file was found alongside it. The data was apparently found in a spot that would have been reserved for Larvitar.
As such, the palette appears to be belonging to Larvitar, hence why the color is all messed up.
It is not known whether it was intended to have that particular coloration, or Gen 2 Cyndaquil's coloration, or neither.

It appears these backups are from the time period where GS betas were still using the Gen 1 engine (think: Spaceworld '97).

Given it was thought to be an Ice or Steel type, I came up with a coloration with those types in mind.

My fan Ice or Steel coloration		Beta Cyndaquil Backsprite