Gengar and Clefable have, through trademarking and beta art, been proven to have been some of the earliest Pokémon made. (Coincidentally, they are also some of the first Pokémon when you list the original 151 by index number, with Gengar appearing shortly after Clefable.) Also, Clefairy was originally going to be the mascot of Pokémon (instead of Pikachu), so it was going to be a pretty important Pokémon.
Gengar is known as the "Shadow Pokémon", but what exactly is it a shadow of? Take a look at a picture of Clefable, and then one of Gengar. Their body shapes are almost exactly alike, with the only major difference being that Clefable has a curl for its hair and tail, while Gengar has spikes. However, the shadow of an upright spiral like Clefable's tail would only be a line, or a spike.
Now look at their typings. One of normal-type, and the other has ghost-type. Due to their immunities, neither of them could hit the other with STAB attacks, since they'd just be immune to each other.
Have you ever tried to hit your shadow?