What creature is Tom Bombadil?

[Raynor]

In letter #19, Tolkien calls Bombadil the "spirit of the (vanishing) Oxford and Berkshire countryside", an obvious out-of-story referrence. More importantly, he says of him:

Originally Posted by Letter #144

And even in a mythical Age there must be some enigmas, as there always are. Tom Bombadil is one (intentionally).

Concerning his resistance to the ring, the closest Tolkien came to explaining it is this (to my knowledge):

Originally Posted by Letter #144

...both sides in some degree, conservative or destructive, want a measure of control. but if you have, as it were taken 'a vow of poverty', renounced control, and take your delight in things for themselves without reference to yourself, watching, observing, and to some extent knowing, then the question of the rights and wrongs of power and control might become utterly meaningless to you, and the means of power quite valueless. It is a natural pacifist view, which always arises in the mind when there is a war. But the view of Rivendell seems to be that it is an excellent thing to have represented, but that there are in fact things with which it cannot cope; and upon which its existence nonetheless depends. Ultimately only the victory of the West will allow Bombadil to continue, or even to survive. Nothing would be left for him in the world of Sauron.

with another occasion, I commented that:
Interesting that Tom is compared to Rivendell; there dwells the greates loremaster of Middle Earth at the time. Another interesting parallel is that both Tom and Elrond are healers in their own way (Tom at least of land). The elves are also said to have the subcreative faculties in the highest degree among all mortals, and the peculiarity of subcreation/Art is its lack of possessiveness, which also seems to define Tom as well. While both the Elves and Tom enjoy lore, the Elves seem to be the closest to perfection in creating, and Tom seems to be closest to perfection in attitude (a chapter where the elves lack, since they can be seduced to a kind of minor Melkorism, to be their own masters in Arda - cf. the Athrabeth). Just as Men/Elves represent the two facets of Mortality (letter #181), Elves/Tom may represent the two facets of the perfect archetype.

I would also compare Tom's 'invulnerability' to the ring to his simple and pure nature, which is apparent in hobbits too - Tolkien commented in letter #109 on the "refusal of their nature and reason at the last pinch to compromise or submit".