We know that Tolkien heavily borrowed ideas from Norse and other mythologies, since Middle-Earth was meant to be the missing myths for England.
Was there a figure similar to Tom Bombadil in Norse or any other myths that Tolkien was using as basis for his work?
Similar meaning - old (oldest) creature, odd and somewhat OUTSIDE the normal pantheon, deliberately vague as to who and what it is. Impervious to supernatural evil (Tom is immune to the Ring) but doesn't fight it.
Please note that Tolkien apparently state that Tom!=Eru in the letters, so "top level world creator living in disguise" figures don't seem to match.