Unknown beings!?

[Zigûr]

Originally Posted by Ulvenok

None can defeat a valar, Melkor was never defeated just thrown out of the world. So no a maia wouldn't be able to defeat a valar.

Given that none of the Valar fought in the Host of the West during the War of Wrath surely someone of sub-Valar stature must have been the one to personally best Morgoth at the end of the First Age. I always assumed it was Ëonwë himself who did the deed, although given how much Morgoth had weakened himself by that point he may be a bad example.
I'm not averse to the theory that Ungoliant was a being whose origin lay in the Discord of Melkor; evidently there were beings which operated outside the conventional Vala-Maia-Elf-Man "power structure" of Arda.