One of the wonderful things about RPGs is that they give us a mechanism to deliberately categorize the world; this is a ghost, while this is a specter, and this other thing a wraith. Sure, none of them exist, but now we know exactly what their differences are and what they can do. It’s the modern version of counting dancing angels on the head of a pin, I suppose.
But it has some downsides, too: there are explicit lists of monsters, but also the leaders of monsters. For all the worlds of D&D, there is one canonical Hell and it is named Baator and ruled by the Lords of the Nine and this is Stygia and this is Cania and this the dark heart of Nessus. It’s all actually interesting stuff, with a lot of resonance and echoes, but it’s also static and dead. Your campaign world is your own, wide and varied with hills and forests, but it makes Hell itself small and sad and prosaic, stuck in the ruts laid down twenty, thirty years ago. It shifts; the Harrowing swapped out some of the faces for others, but still, we publish products specific to this view.
Too, each of those products needs to fill page count, so there’s a lot of repeated flavors. Tiamat and Demogorgon need to work hard to differentiate themselves; while their physical forms are fairly different, they’re both reptilian-aspected polyfurcated evil snake things. It’s actually kind of odd that the five-headed Queen of Greed is so on-message; surely her heads should be plotting against themselves, as Demogorgon’s component parts, Aamuel and Hethrediah, do? So Tiamat gets greed and Demorgorgon dissociative identity disorder; we distinguished them. This duplication functions as a spur to creativity, and it means there’s lots of different high-challenge potent beings out there, but… meh?
Tiamat is one half of an ophidian dyad, the other half of which is a bright and shining exemplar to her perfidy. Sort of like Asmodeus and Ihys, if the dark of late-90s Planescape is to be believed! Sort of like Eberron’s celestial dragons (though they add the middle sibling, of course)! I guess that’s all the paired one-good-one-bad snake creatures I can think of, but isn’t that enough?