I think Mac is King Arthur.
I read somewhere that Butcher was a fan of Tim Powers who wrote Drawing of the Dark. The theme is old gods wandering the earth in different forms. Similar to how the older gods exist in different forms in Dresdenverse. The story revolves around a magical beer “the dark” that can only be brewed every few hundred years.
In the book, Arthur is the protagonist, not the brewer. But ever since Small Favor where Harry faced down the Gruff and Mac brought out the dark beer, I’ve been convinced of this. Plus, if Merlin and Excalibur exist in the Dresdenverse then King Arthur obviously exists too.
Prove me wrong. ;-)