Just read a post pointing out that the only fires in the game that are already lit and kindled when you arrive are the three tended by firekeepers (Firelink, Anor Londo and The Fair Lady), and the one where the ancient dragon sits. As we can't kill it we can't see if it drops a firekeeper soul, but it looks as though it's sitting on a nest so might be female, and the fair lady shows that you don't need to be human to become a firekeeper (having become one after turning into a chaos demon).
Given the Firekeeper Soul descriptions of the non Named Firekeepers It might be an apt observation.
Each Fire Keeper is a corporeal manifestation of her bonfire, and a draw for the humanity which is offered to her. Her soul is gnawed by infinite humanity, and can boost the power of the precious Estus Flask. Reinforced Estus Flasks capture denser Estus, allowing for increased restoration of HP.
Since the Everlasting Dragon is at the Bottom of an Arch tree in Ashe Lake, which is Frozen in the time of Dragons, it can be safe to assume that the Firekeeper would be an enormous Dragon.
As for the other Firekeepers that we never met but can claim their souls, It might be safe to assume the One we find in New Londo was or became a Dark Wraith, and was drowned when it was flooded. The One in the Duke's Archives more than likely became/was turned into a Piasca. While the one in the Undead Burg more than likely was human/Undead and killed by Lautrec which was why he was locked up there.