Boldogs, Gothmogs and Sauron's New Clothes

[Neithan]

Perhaps these other creatures were of the same race as Ungoliant, although lesser than her, and they too had crept in from the shadows that lay about Arda.

Interesting, I had always assumed that these spiders were just creations of Morgoth just like the Orcs and other things but what you say is possable.
Also, Tolkien never said for sure that she "descended from the shadows about Arda" only that the Elves believed that she did. So she could be just about anything.