I believe 2.0 story showed a hint on how that was possible being
The 2.0 WoL was the one who gave Cid his goggles back when he or she has yet to be a physical being in the world yet.
Sometime after that event the WoL manifested into the world and what life and time they spent in the world they had until the beginning of 2.0 is up to the player's imagination.
With this there are basically endless possibilities with the WoL's Origin from being a person from another world, reincarnation into this world, natural residence of the world, and any toher cliche origin story a person can think up.
As they say sometimes the best Origin option is the one the Player can freely make up themselves.