Who or What is Tom Bombadil?

[Legolas]

always find it fascinating when the question of Bombadil comes up, that no-one asks who Goldberry is,

This came up on the previous page, actually! Here it is, in case you missed it.

Here's what was said, by Bethberry:

On the other hand, does the problem or enigma lie only with Tom? I find it very interesting how often people have difficulty understanding or accepting Tom, yet very few seem to ask who or what Goldberry is.

Do readers have an easier time accepting a female mythic earth character than a male one? Or is it that Frodo's infatuation with Goldberry provides an adequate and understandable explanation of her function? (Why is it that she can control rain, but not snow, for instance?)

My response was...

Even if Goldberry's nature was as unexplained as Tom, I don't think she would have attracted the amount of speculation Tom has. The point with Tom is not that he is male, but that his physical acts (and subsequent conclusions drawn about his mentality) show that he is quite an oddity - he skips about through the woods carelessly, and is completely unmoved by the Ring? He's not just an unexplained character. He's an unexplained character that leaps out and thumps the reader in the eyeball.