[mhagain]

Oh, Ungoliant's easy. Let's look at the evidence. Exhibit A, m'lud:

in ages long before she had descended from the darkness that lies about Arda, when Melkor first looked down in envy upon the light in the kingdom of Manwe

So we need to find the time "when Melkor first looked down in envy upon the light"; let's read the Silmarillion: Of the Beginning of Days. Exhibit B, m'lud:

...far off in the darkness he was filled with hatred, being jealous of the work of his peers, whom he desired to make subject to himself. Therefore he gathered to himself spirits out of the halls of Eä that he had perverted to his service, and he deemed himself strong. And seeing now his time he drew near again to Arda, and looked down upon it, and the beauty of the Earth in its Spring filled him the more with hate.

Note the wording. "Jealous" - "envy". "Looked down". That can't be an accident. And this was just at the time before Melkor assailed and destroyed the Lamps, so yes, he was looking down on the light too.

So that's when Ungoliant came into the world.

Now let's read Of the Darkening of Valinor:

But she had disowned her Master, desiring to be mistress of her own lust

So Ungoliant originally had a Master, but who was he? Let's read the same chapter in Morgoth's Ring, where there's a bit more dialog that CT diidn't use in the published Silmarillion:

Thrice fool: to leave me first...

OK, so her Master was Melkor, she came into the world when Melkor overthrew the Lamps, and that was the time when Melkor "gathered to himself spirits out of the halls of Eä that he had perverted to his service".

So that's what Ungoliant is: one of those spirits.