Flareth

Flareth, also known as Flarith, is a PokéGod. It was thought to be the evolved form of Flareon.

Origin
Like many other Pokémon, Flareon was thought to have another evolution. This led to the creation of Flareth. Interstingly, no other evolution of Eevee had a widely circulated evolution. In addition, the Mist Stone was never stated to evolve Flareon into Flareth, unlike most other evolutions of current Pokémon. Most of the time, Flareon was not needed to obtain Flareth either, making it seem less related to Flareon than with the other PokéGods and their pre-evolutions.

The reason for Flareth being the only evolution of an Eeveelution, different from other evoltuions of current Pokémon and not being attached to its pre-evolved form may be because it is a Fire-type. Fire was a very uncommon type early on, leading to Fire-type PokéGods covering more than one Fire-type since they are supposed to be even rarer. For example, Flareth was also occasionally stated to be the evolution of Charizard, Arcanine and Rapidash. Flareth was also sometimes said to be the true name of MissingNo. or to actually be Ho-oh, another Fire-type PokéGod.

Flareth may have originally been created as a name for Honōguma, a Pok émon created during the development of Gold and Silver. This name was not widely circulated, causing fan made names to appear such as Pikaflare and possibly Flareth. Eventually, these two could have separated into separate PokéGods, with Flareth being attributed to Flareon because both have "Flare" in their names. However, if this is true, the other Fire-types could easily have been attributed to Flareth as well, which gives reason why Flareth had so many different pre-evolutions, but no connections to them when being obtained.

Celadon Department Store
The mainly circulated way to obtain Flareth was to talk to the thirsty girl on the top floor of Celadon Department Store. It was required that you bring Articuno, Zapdos and Moltres, though some codes required three Geodude in the remaining slots of your party. This is strikingly similar to the party needed for Mr. Psychic and Charcolt. However, other codes described bringing four Moltres to the girl instead. You would then need to talk to the thirsty girl one hundred times and on the one hundred and first try, she would become annoyed and give you Flareth so that you would "leave here alone." If you talked to her again, some codes would say she would threaten you. Sometimes, variations would have additional requirements like all 150 Pokémon or to use the Itemfinder next to the Unknown Dungeon to locate a bottle or note leading to the girl.

Trivia

 * The sprite used for Flareth in the image is a scaled-down version of Ridley's artwork from Super Metroid.