As for the comment above regarding making unique creatures in to races, it makes sense as a source material gets adapted into a gaming setting.
Unique creatures are good for fiction, myth, and legend.
However, in gaming, unique creatures create limits for DM's. If a party kills a unique creature, then they've written themselves into a corner because they can't fight a second one. . .or fight two at once. If the campaign is designed with that in mind up front, it's not as much of an issue, but for a general-purpose D&D setting that will have many authors, and be played by countless groups across the world for many years to come, it does bring more limitations than options.
In multiple campaigns by one DM in a shared continuity), unique creatures create the question of who really killed who. For crossover settings like Planescape (or Spelljammer), having many different worlds with only a small number of named beings raises issues (both PS and SJ imply dozens, maybe even hundreds of worlds, and there are only 6 Class VI/Balor-type demons? That's a lot of worlds to tempt and places to cover, and a lot of worlds where heroes could arise to eliminate the few that exist.)
In my opinion: as a game setting design issue, making famous or notorious monsters like the Balor, the Marilith or the Medusa into a race of beings can make for a better setting, especially for an "off the shelf" setting.