[HairOfTheDog]

I could see Old Man Willow as an huorn (whew, do I have it right now?) before I could see him as a Maia.

Now Bombadil, that is a different story.

Suddenly he stopped. There was an answer, or so he thought; but it seemed to come from behind him, away down the path further back in the Forest. He turned round and listened, and soon there could be no doubt: someone was singing a song; a deep glad voice was singing carelessly and happily, but it was singing nonsense:

Hey dol! merry dol! ring a dong dillo!
Ring a dong! hop along! fal lal the willow!
Tom Bom, jolly Tom, Tom Bombadillo!

[JenB]

I just get the sense that Old Man Willow is something sinking down from a higher stage, rather than a tree becoming more sentient.. something that might not always have been evil, but over time has lost interest in anything except its own hungers and desires.