[Mithadan]

My preference is to interpret Tolkien's Legendarium as internally consistent (where possible). So, the speaking peoples/entities are made up of Ainur (including the Valar and Maiar), Elves, Dwarves, Men, and Ents, though some of Morgoth's corruptions may have been interbred with beasts. To me, all of Tolkien's speaking peoples fit within these categories or some combination thereof. So, in my view, Ungoliant was a rogue Ainu, recruited initially by Melkor/Morgoth.

When one discusses Tolkien's late musings and philosophical writings, those not published in his lifetime, we run into the concept of "canon." What did Tolkien actually decide upon as his "final" conception, versus what he was creatively fiddling with. Canon is in the eye of the beholder. In my interpretation of his Legendarium, Tolkien's notes and writings about Arda always being round and his thoughts about an expanded version of Ea fall into the latter category. Regardless, if one wants to shoehorn them into his final conception, then Ungoliant could be an "alien Ainu" or merely an Ainu corrupted by Melkor who came to Arda.