I think it is more than likely that Ungoliant was one of the Ainur who followed Melkor in his discord.
In the Sil it states (with the qualifier "The Eldar knew not whence she came; but some have said...) that she descended from the darkness around Arda, "when Melkor first looked down in envy upon the Kingdom of Manwe, and that in the beginning she was one of those that he corrupted to his service."
As for Melkor being afraid of her (suggesting that she was more powerful), remember that "the Valar drew unto them many companions, some less, some well nigh as great as themselves." Also, she had just sucked the light out of the Trees, and drained the Wells of Varda, and "had grown great, and he (Melkor) less by the power that had gone out of him..."
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only that Part from which he came
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