I don't recall where I got this impression, but I thought that the Erlking was the Summer King (Different court than the Summer Court), and he was balanced by Kringle, who is the Winter King (Not part of the Winter Court).
That's a fantheory I've read, but IIRC WoJ has explicitly denied it. In particular, there is only
one Winter Queen & Summer Queen, and there is no corresponding singular King for either. The Erlking is
a king (who is Summer(ish)), but not
the king -- except of the Goblins.
I don't even think Kringle is a "King" per se; but (as a part of Winter) he is in fact subject to Mab (something she's careful about, since the current Kringle is only part-time, and the rest of the time he's... rather more than that).