“Maybe there’s somebody else I haven’t mentioned, but who?”
Well, Maggie, but that seems way out there, since she’d have to be crossdressing or something to account for the people calling her a “he”.
Another one I think could be a remote possibility would be Grandpa Sam Tuttle, if he never really died. We haven’t seen him, but we have learned a bit about him. Also, if the Yellow King is “without time”, could that be some reference to Sam Tuttle cheating death? Far-fetched, but I guess let’s throw it out there if we are covering all the bases.
Also, how about the black preacher we met for one episode. Doesn’t seem likely at all, but I think his church was connected to Tuttle, he knew about the “devil traps” from his childhood, and Tuttle’s housekeeper said that Sam had “all kinds” of children, which implies to me, in an “Old South” context, that he wasn’t above miscegenation. A black man being a Tuttle spawn running things would be unexpected.
btw found this on the web
"...the picture in Dora Lange’s house of a young blond girl surrounded by the “Five Horsemen” looks an awful lot like young Audrey; when Audrey was a child in ’95,
http://www.thedailybeast.com/articles/2014/03/05/craziest-theories-of-how-true-detective-will-end-killer-marty-the-five-horsemen-and-more.html 
maybe this is what was alluded to in the spoiler I posted previously-- “a seemingly arbitrary detail in a photograph, along with a homeowner’s old tax records, provides the pair with a vital new lead in their 17-year-old case. The two men pursue the lead to an apocalyptic confrontation.”
I rewatched the show and (I think it was episode 3) when the pic was discovered) I was surprised they didn't ask about it.
I just assumed it was a younger dora lange. But maybe not.