I like the thing about pre-Vala-pantheon... Nobody knows where U. comes from,
she is a mystery (cannot possibly have been created by Eru?) I think the
inspiration must have been something like the mythic dragon Pytho at Delphi,
the guard of the Mothergoddess Ge, that Apollo defeated when he took over the
oracle. Something frightening for the followers of the new, well-ordered
pantheon of the Greeks (comparable to Tolkien's combination of pantheons, where
he has a bit of the indoeuropean group of gods, each with his/her speciality
(functioning a bit like mighty angels) with a Christian main god placed
above).