There is no canonical answer to this question
In no published source that touches on the eladrin, and there really aren't that many, is there a canonical answer pre-4E (and that one is... well, let's not comment on that). The celestials have always gotten less attention and detail than the fiends, for obvious reasons. We actually don't have a firm origin, or even suggested origins, for any of them. Notes on the eladrin identify the notion that they might be elven spirits but also reject it as an unfounded rumor - and not in the usual way of winking at the reader and saying "but maybe..."
Here's what we do know
The eladrin originated from a time known as the Age Before AgesFC1. beings of chaos and good who allied with the obyrith known as the Queen of Chaos in the ancient war against the forces of Law. When Miska the Wolf-Spider was banished by the Rod of Law2E:RoSP,3E:FC1 and the forces of Chaos were sundered, the eladrin struck out at their former allies and so devastated the obyriths that they were unable to fight back against a similar betrayal by the tanar'ri.
The eladrin are led by Queen Morwel, who resides in the Court of Stars (both a demiplane attached to Arborea as well as the metonym for the eladrin "leadership" and nobility). She has been the only eladrin leader for as long as anyone is awareBoED, although her consorts have changed over the millennia.
Ruins found in Mithardir (also known as Pelion) as well as encounters with a mysterious and rare race of albinistic "elves" in and around said ruins suggest that the eladrin were either originally very different from what they are now or else were preceded by beings who wielded great and terrible powers, and that whoever erected those now-ruined buildings likely had a hand in the foundation of the cosmos as known2E:DG. Certainly legends speak of a time when Mithardir was forested and a race of titanic beings, perhaps lost deities, ruled the now-dead layer of the planeMotP.
Source List
2nd Edition Dead Gods (Planescape adventure); Planes of Chaos (Planescape accessory); Planescape Campaign Setting; Planescape Monstrous Compendium II; Rod of Seven Parts (D&D adventure); Warriors of Heaven
3rd Edition Book of Exalted Deeds; Fiendish Codex I: Hordes of the Abyss; Manual of the Planes; Monster Manual; Planar Handbook
Not an exhaustive list, provided mainly for corroboration of major points or for further reading on topic of interest.