You are right that Tom must be a Maia. However, I think Tolkien just put him in there for fun. Its been said that Tom Bombadil was the spirit of the English counrtyside that Tolkien loved but was being threatened by industry. The actual name "Tom Bombadil" and the specifics of his dress were based on one of the dolls of Tolkien's daughter. I love how Tolkien put this amazing character,who is not important in the plot, right into the story.
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