In the Wicked series written by Gregory Maguire (the ones that the musical is loosely based off), Maguire uses the original Oz series author Lyman Frank Baum’s intials to create a name for the previously unnamed Wicked Witch of the West.
L F B - El FA BA -> Elphaba
L. Frank Baum has been dead almost 100 years and never had a name for the Wicked Witch of the West seeing as she was only in his first Oz novel.
Disney used a different name of Theodora for their Wicked Witch of the West because it has no affiliation with Wicked the Musical or Gregory Maguire’s book series.
Therefore, we do not know which way Once Upon a Time is going to spin this Wicked Witch of the West and her back story. Be prepared that she most likely will not be named Elphaba seeing as ABC does not have the rights to name her that.