Doomsday

Doomsday was a PokéGod. It was thought to be the evolution of Doomsay.

Origin
Doomsday is thought to have been created just amongst the PokéGods rumors. It later gained a pre-evolution named Doomsay, which may have been created due to a misspelling of Doomsday's name. It was thought to be number 158 in the Pokédex, though some sources stated that it was number 152. Though no information was officially stated in codes, it was generally thought that Doomsday was a horrific, deadly and powerful ghoul-like Ghost-type. This led to people fearing trying to get Doomsay as other regular PokéGods could mess up graphics or delete save files, and these PokéGods could potentially do more than that.

Doomsday may also have a basis as a name for Houndoom before Houndoom was a real Pokémon because both were the second and final stage in their evolution line, had an evil connotation, and contained the word "doom" in their name.

Ghost Stone
One of the methods thought to obtain Doomsday was to use a Ghost Stone on MissingNo. and Gengar to fuse them into Doomsday.

Gengar
It was often speculated that Doomsday was the evolved form of Gengar to complement the fact that Doomsay was thought to evolve from Haunter. Haunter and Gengar were two of the three Ghost-types in Red and Blue, making it easy to see why Ghost-type PokéGods would be thought to be related to them.