Pokémon has spawned a number of fan theories over the last two-and-a-half decades. Somehow, neither Game Freak nor The Pokémon Company has presented much evidence to confirm or debunk these theories, so they end up becoming many fans' headcanon. They range from Ash and Pikachu being dead to the Legendary beasts starting off as Eeveelutions. One such longstanding theory is that Gengar is actually the "shadow" of a Clefable.
The Gengar-Clefable-shadow theory is actually fairly prevalent. There seem to be all kinds of evidence to back up the claim, including their appearances, types, and Pokédex entries. However, not everyone agrees with this theory. There are those who would put forth that Gengar is not a shadow but is instead a dead Clefable. And then there are the creators of Pokémon who may just outright deny either of these theories. Since this is all just speculation, fans are free to decide for themselves what to believe so here's some of the clues that may lead to their connection.
First of all, there are all sorts of reasons to suggest that Gengar and Clefable are two sides of the same coin. They both have similar physical characteristics, including their pointed ears, their backs, and their body shapes. Since Clefable is a Normal-type Pokémon (up to Generation V) and Gengar is a Ghost-type, neither can harm the other, thus reflecting the connection between the person and their shadow. Gengar's Pokédex entry calls it the Shadow Pokémon and says that it likes to act as others' shadows at night. There's even the possibility that Gengar's name is derived from doppelgänger, which further feeds into its status as an imitator. With things like this in mind, it stands to reason that Gengar and Clefable are related as Pokémon and shadow.
On the other hand, some theorists believe that Gengar is a dead Clefable, and there's evidence to back that up, too. Clefable's type has been retconned to Fairy as of Generation VI. This change makes it so Clefable and Gengar can hit each other with their attacks, so the Pokémon-to-shadow relationship doesn't exist anymore.
Furthermore, when fans refer to Gengar as Clefable's "shadow," it can be seen as synonymous with "evil half." However, if Gengar really were an evil version of Clefable, there's a type that would suit it better-- the Dark-type. The literal translation for Dark-type from Japanese to English is "Aku type" or "Evil type." If Game Freak had meant for Gengar to be an evil anything, they could have given it this type back in Generation II. If Gengar is not an evil Clefable, then that's another tie severed between the two.
However, there are still ways to connect Gengar and Clefable. They both still look similar to one another. Furthermore, while Clefable is no longer a Normal-type, Gengar is still a Ghost-type. Even if Gengar cannot be an evil shadow, it could still potentially be the spirit of a dead Clefable.
As things stand, there is no concrete evidence to confirm or deny that Clefable and Gengar have any relationship with each other. However, this also means that fans are free to come up with their own headcanon. Gengar can be a shadow or it can be a ghost; either works for different reasons.
