Scratch

Gengar and Clefable have, through trademarking and beta art, been proven to have been some of the earliest Pokemon made. (Coincidentally, they are also some of the first Pokemon 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 Pokemon (instead of Pikachu), so it was going to be a pretty important Pokemon. Gengar is known as the "Shadow Pokemon," 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 types. 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?