For instance the relationship between Gengar and Clefable....
Now this is all fan made speculation, nothing has been proven by game freak or nintendo. But have you ever noticed how similar looking the two are? Gengar's species is the shadow pokémon....
Which has led to speculation that Gengar may be Clefable's shadow, this does sound believable, as their typing is ghost/normal, so its not like the two can hit each other, just like you cant hit your shadow. They were also some of the first pokémon to created, due to betas. Clefable was also meant to be the original mascot for the pokémon franchise, opposing pikachu, this means that Clefable would be considered an important pokémon, and this story may have meant to be incorporated into the anime?