[Riftmaker]

I would agree with Six. Offspring of Maia are not Maiar. However, they usually have some "special" abilities not normally conveyed by their other half.

Luthien's magical song for instance.

This brings up the question of the relationship between Melkor and Ungoliant. I honestly don't believe that Melkor would "ally" himself with a lesser being, especially on equal terms. Melkor and Ungoliant were, basically, allies, and maybe even friends in some twisted way.

Of course this can all be related back to good ol' Tom. If Ungoliant isn't a Maia (which is entirely possible) just some dark, evil, evil monster, then what is Tom? We're using the same rule on both of them, the process of elimination. They aren't anything else, so they have to be Maia.

I can't think of any other creature that can be placed into this category. The origins of every other living thing are clearly defined in JRR's texts. This is very odd. If anyone else can come up with another critter other than Ungoliant and Tom that aren't defined at all, please post.