The Erlking before joining the hunt The Erlking can walk over water which most faes avoid doing because it weakens them; this could be simply because he is powerful enough to afford it. There is also the fact that he is a giant that happens to like to hunt. I suspect that before becoming the Erlking he was Orion the Hunter. He was a son of Poseidon and Gaea, who had the ability to walk on water who also became companion and possibly lover of Artemis, goddess of the moon and the hunt. In his death she turned him into constellation. Maybe this represents how Artemis turned him into the Erlking.

I'll admit, what I have isn't much to go but it seems like an interesting possibility, since many old gods like Odin have joined the fae courts.