The identifiable predecessor of the Plane of Faerie, the Seelie Court, appeared earlier than that, as early as the Planescape product Planes of Chaos. Even at that point, it wasn't part of the traditional Outer Planes, but was accessed from various planes like Ysgard, Arborea, and the Beastlands. Already at that early stage, it was something outside and different.
A key difference between the Seelie Court and the Plane of Faerie is that there is nothing making it an overlapping companion of the material world. The 2e Seelie Court isn't particularly distinguishable from, say, the 1e
Manual of the Planes's realm of Olympus, a divine domain nominally on the Plane of Olympus/Arvandor which sits at the top of a conduit that connects to Hades and to every material world that worships the Greek pantheon. The idea that there's a fairyland plane with a special next-door relationship to the material world isn't introduced until the 3rd edition
Manual of the Planes details a Plane of Faerie coexistent with the Material Plane, "and a location on the Material Planes matches with a Faerie duplicate" (p.210).
It's that coexistence which is the key conceptual step in moving from something like the Seelie Court, a slightly exotic realm in the Outer Planes, to the Feywild, a bright parallel of Shadow. Because from the very beginning in 1980, we read, "The
Plane of Shadow co-exists with the
Prime Material Plane, and is the result of the interaction of that plane and the
Positive and
Negative Material Planes".
Until there's a fey plane cosmologically equivalent to the Plane of Shadow, a fey equivalent of the shadow dragon doesn't have any particular logical force. I mean, sure, you
might have a dragon native to any given Outer Plane or realm thereof, but only if you have a particular idea for one. And until Faerie gets promoted from an optional plane mentioned near the back of the
Manual of the Planes to something canonically equivalent to the Plane of Shadow, it's unlikely to get published in any case.
So, in 1983, the faerie dragon and the shadow dragon are published on opposite sides of the same book leaf, and have nothing in particular in common, because there's no reason for them to. It'll be eighteen years before there's a plane with the necessary attributes to be a fey equivalent of Shadow in the game, and another seven before it's part of D&D's standard cosmology.
