This is one of my favorite and most interesting fan theories. Many fans say that Gengar is just a shadow of Clefable. Think about it! Clefable and Gengar are similar in many ways. Same hands, same ears. Clefables wings=Gengar's back spikes. Gengar is classified as a Shadow Pokémon. Think about it!!!

Clefable and Gengar Front and Back Sprites (From Pokémon Emerald Version)
Spr 3e 036
Spr b 3r 036
Spr 3e 094
Spr b 3r 094