Very much agree with both g33k and Bad Alias here.
The reasons there is confusion on this topic is there are WOJ's that say that the Erlking is a Summer King, other later WOJ's say he is a King of the Wyld Fae, and that it doesn't matter as that there is a reason Mab rules the worst of the worst and not the Erlking, despite him previously calling them 'peers'.
Early WOJs say his origins are on the Summer side, but I think it it more complicated than that. Kringle's origins are likely as not Summer in nature, yet he is thought of as the 'Winter King'. It is more about which side the Wyld Fae choose during conflicts between the major Fae powers (i.e. Summer and Winter), just like mercenary lords in a war between two Monarchs.
And remember, the Erlking wasn't always merely the Erlking. Just like Kringle that is more likely just one of his masks. Another was hinted at during his first appearance in Cold Days when Eldest Gruff referred to him as "Lord Herne" as in Herne the Hunter from Celtic mythology. These are gods wearing mantles in order to preserve their active power in the world, immortals find change so very difficult. The Queens of Faerie have long been guessed at as being someone (or a group of someones) other than their "normal" mask/mantle.