Unknown beings!?

[Inziladun]

Originally Posted by Ulvenok

It's a bit hard to put them in that ainu category, because they have both shown to be far more powerful than ordinary maias like Sauron and Melian etc. The valar are specifically those who sang the great music and neither Ungoliant or Tom were with them. One can imagine them to be similar to the valar but not really them, because they are numbered.

While all the Valar were of the Ainur (as were the Maiar), it wasn't only they who took part in the Music. Some of the Ainur remained with Ilúvatar outside Arda.

Thus it came to pass that of the Ainur some abode still with Ilúvatar beyond the confines of the World; but others, and among them many of the greatest and most fair, took the leave of Ilúvatar and descended into it...Therefore they are named the Valar, the Powers of the World.

Silmarillion Ainulindalë

The Valar were only the greatest of those known to the Elves. They did not know the number of the Maia.

Additionally, Bombadil at least was apparently not the match of Sauron. That was seemingly the consensus reached at the Council of Elrond when the idea of sending the Ring to him was rejected.