Apr 8, 10:22pm
This puts me in mind of a couple moments where CS Lewis also offers a fleeting peek at a vast, teeming underground world beyond the knowledge of the surface dwellers (in the Silver Chair and in Perelandra). Not the only time I've felt like these two friends settled on parallel approaches to a subject; each also performs a similar maneuver to institute a pantheon of gods while making just enough mention of one shadowy boss god above them to maintain their clean conscience as monotheistic authors, for example.
As to what these beasts were and how to explain them and bring them into alignment with the rest of the mythology, though, I think the answer must be much the same as with Tom Bombadil. Some things are meant to be mysterious; I think the author, while he would appreciate all the loving attention to his mythos that his readers bring, would nevertheless say that some of his riddles simply won't benefit from closer and closer scrutiny. He who breaks a thing to find out what it is, and all that.
Intellectuals in the eighteenth century were much exercised by the idea of the Sublime, and there was a vague but extensive written dialogue between writers trying to grapple with a fairly nebulous idea put forward by a classical writer called Longinus. And there was a lot of variation in how these scholars and blowhards tried to explain the idea, but it was at least something more specific and subtle than just "sublime" meaning "really great." To borrow from Edmund Burke, if I remember right, some characteristics of the sublime were that the sublime is fearsome, that it is associated with large things and high things and old and faraway things, and critically for our purposes here, that what is sublime must never be fully revealed. It is always obscured to some degree.
This last characteristic is all over Tolkien's approach (and I can't doubt that he and Lewis were well aware of all these writings, more than I am surely). Tolkien loved to say that in good stories the reader could hear "echoes of the horns of elfland." And Middle-Earth is loaded with them; there is a suffusing sense that an ancient, beautiful, altogether higher and better world is slipping away as the elves pass into the west, as the ents dwindle, as the scions of the Numenoreans grow fewer and live less long. And the threats, too, are kept in the shadows for the most part: Sauron, the prime mover of all the action and arguably the single most important character, is never seen directly in the trilogy, the Nazgul are hooded and only rarely and fleetingly visible as what they are. Again and again it happens that some character with a lot of veiled power stands up a little straighter and seems to grow taller and more terrible for a moment. He loves this sort of imagery; the examples go on and on. But it is important to his idea of a fairy-story that it is animated, not by the horns of elfland, but by their echoes. The sense that there is more around the corner, if only you could crane your head to see it, makes the actual story seem higher and deeper. The end is never final. There is always more to discover, another layer of the onion. It's like real life.
So, to me it seems almost axiomatic that at the deepest point anyone digs in Tolkien--no matter how deep that is--inevitably, what they will find there is a signpost saying that there's much more to see if only they would go yet deeper.