Rider in Mirkwood

[Sharkû]

To your well observed ontological interpretation of the Mirkwood-deer, Underhill, I have to say it is hard to add anything, so I will just throw in some cross-references I noticed.

The “Elves in The Hobbit perform the same function as the stag usually does in myth – they lure the heroes off the known path and into a mystical, enchanted realm” (Underhill).
This is obviously the main point about this encounter in Mirkwood, the confrontation of the other- (or maybe, as we will see, nether-) world with the corporal world. Here it is interesting, by the way, that the dwarves, which normally stand for a certain otherworldliness themselves, are on the purely physical level of things.

One trait of Middle-Earth, and The Hobbit no less, is that while mythological themes, topoi, can be applied, it can, due to its nature, not be associated with a single complex of real world myth, if at all with a concrete one.

In this example, I found two myths in particular striking, one from the Celtic mythology, one from ancient pan-Germanic sources, including England.
If we reduce the passage of the deers, and the ensuing encounter with the Elves, to the striking quotes, these become more obvious:

There was a flying sound of hooves […]a flying deer […] charged […] the shadows swallowed it up […] blowing of horns in the wood and the sound as of dogs baying far off […] the noise of a great hunt going by to the north of the path, though they saw no sign of it […]hind and fawns as snowy white as the hart had been dark […] a woodland king was there with a crown of leaves […]on a chair of carven wood. On his head was a crown of berries and red leaves, for the autumn was come again. In the spring he wore a crown of woodland flowers. In his hand he held a carven staff of oak.

The appearance of the hunted stag and the Elven court, and how they look to the perception of the dwarves, remind, for one, of the Wild Hunt, a central item not only in Anglo-Saxon mythology.
Even though the word ‘fly’ is in Tolkien’s works always also used in the sense ‘flee’, and this undoubtedly is the case here, too, it seems once again likely that an ambiguity with ‘move through air’ might have been intentional, for the passing of the beasts and the hunt evoke this imagery, too. This is also the very case with the Wild Hunt, led by the Grim Hunter, King of the Dead and Spirits, ultimately, Odin. The black and white deers hunted may in this analogy -- which I admit is no more than just an interesting parallel, not an interpretation, and by no means an allegory, to gainsay such accusations aforehand – both be no more than beasts, or also women or spirits clad in beastly hue, for both we know as being possible as well in Middle-Earth as in the myth of the Wild Hunt.

What appears especially interesting, is how the Hunt meets the company. While Thorin and the other dwarves are eventually taken captive by the King, Bombur, who is standing in the way of the Hunt, is rewarded with an insight into the otherworld, and transcendence the way he imagined it, by the dreams he receives after getting hit.
The nature of the Wild Hunt as traditioned in legends is similar. Mortals who encounter it are punished, but those who dare to stand in the middle of the way of the passing hunt are rewarded – incidentally, one myth I remember included that Odin slew a stag for the one who stood in his path, and gave parts of it to the mortal, which turned out to be treasures the next morning.

A different legend in which similar motives appear is the Celtic-influenced Ballad of TamLin, a mortal man who, upon hunting in the woods, was taken away by the Elven Queen and her court to dwell in their realm.
Towards the end of the ballad, the passing of the Elven court is described: first would run a black horse, then a brown, and then the white horse of Tamlin. This of course echoes the passing of the deers, and the abduction by Elves is similarly present in The Hobbit as well.

All three accounts, Wild Hunt, Tamlin, and Hobbit, can be abstracted on a higher plain of meaning, which Underhill covered well already.
One sign for the metaphysical nature of the white deer, which somehow may seem more important, is that it cannot be hit by the arrows of three dwarves combined.

What I could add here is that the white fawn of the deer may not only be a solar symbol, or a sign of purity, as he proved, but that the whole constellation of the deers may have an astral connotation – the other two myths have one for sure.
For the white fawn could also be the moon that finally appears behind black clouds, symbolized by the black stag, both moving across the firmament at night. In this way, the white deer has one more attribute that makes it appear as feminine, not only as the prey of the Hunt (which was often a woman desired by the Hunter), but also since the moon itself is feminine in most Indo-European languages.

Such an animate ‘second’ nature of things undoubtedly is one of the fascinating aspects of Middle-Earth, whether the reader examines it more closely, as in this example, or lets it take its effect subconsciously.

Now for that fox in the Green Hills in ‘Three Is Company’…