I think she was an Elda. Explanation:
Facts about Ungoliant
1 - what is Ungoliant? a Spider. What does is her main characteristic as a spider? To weave webs. This perticular Spider seemed to be very good at it.
2 - When does she first appear in the
history of Arda? a few days before the Darkening of
Valinor.
3 - When does she disappear? a long time after the beginning of the
First Age.
4 - What was her action? To draw out the life-force from the
Trees.
Whom are those facts also true of?
Well, think of someone who:
1 - was such a weaver her skills earned her the name Serindë
2 - "died" *cough, cough* a few years before the Darkening
3 - "reappeared" a long time after the beginning of the First Age (see
Morgoth's Ring, the late QS)
4 - Was closely connected to one whose main achievement was to draw some of the life-force from the Trees (in order to make
the Silmarils)
Remind you of anyone? No? Remember the mother of
Fëanor, Míriel? Well, she died of exhaustion after giving birth to her mighty son. Or so the
loremasters would have us
believe, all too anxious to conceal the dreadful truth. Just suggesting.
Very strongly suggesting.
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Seriously, though, I think she was a perverted
Maia.