Hello, Wolflordodin, and welcome!
Erl is wyldfae, neither Summer nor Winter. But his
origins are in Summer.
Kringle is Winterfae... even though Vadderung is not. I'm not sure there's a purely "faerie" entity these days who could/would take up that Mantle... but there very well
could be. Given the amount we know -- and don't know! -- about how the Mantles work, it could even be that there are several entities who pass the "Kringle" mantle around amongst themselves. Vadderung is clearly capable of putting Kringle's Mantle on, and taking it off again, more or less at will.
Oberon is dead (ish). Word of Jim says:
Oberon… well, the guy kind of wound up between Mab and Titania in one of those romantic triangle things, back around Shakespeare’s day. He didn’t make it.
Jim also says:
The Faerie realms just aren’t that structured. It’s more accurate to say that [Kringle] is /a/ Winter King. Or even more accurately, that he is a free Wyld Fae who is of a power level that is on par with Mab’s and happens to neighbor her sphere of influence, and finds it simpler to show up to family dinners during the holidays and make polite than to start staking out boundaries and establishing treaties.
And:
There is no such thing as a Summer King in the Dresden Files faerie cosmology. Mab and Titania need a King like a fish needs a bicycle.
There are a ton of independent rulers of the Wyld, though, much like the Erlking, who is a member of the Winter Court more or less as a sign of courtesy and respect.
DragonsThere's two kinds, apparently. There's the big scaly monsters, like flying dinosaurs with firebreath... these are the emissaries & servants of the
REAL dragons.
Ferrovax is a real dragon. He seldom manifests in the world because he's just too potent a being, reality warps around him. Again from WoJ:
[Ferrovax] isn’t some kind of Smaug hanging around a nice apartment. He’s a Dragon in a more Asian sense of the concept, a semi-divine being who was once given authority over various portions of the mortal universe, and who was responsible for their orderly procession.
There's only a few true dragons left in the world... I don't think Jim has entirely made up his mind on the exact number, or their identities; but it seems to be less than half a dozen.