Spiders?

[Thinlómien]

Ungoliant as a revolting Ainu who didn't participate in the singing and dwelt in the Void before the creation of Arda but AFTER Eru created the Ainur looks to me like the most likely explanation. Or have I missed any suggestions of her existing before Ainur?

This needs clearing up. Not participating? Do you mean that:
* Eru didn't notice her not participating?
* Eru let her 'miss' the singing?
* Eru had no power to force her to sing?
All sound a bit strange to me... I suppose you had something else in mind.

And as Ungoliant as an equal with Eru sounds ridiculous. (No insult meant by this; I just think it doesn't fit Tolkien's image of one God.)