I'm asking because I saw a throwaway reference to this in a documentary about the Tocharians mummies in China. Happy halloween!
Edit: wiki mentions the "Witches of Subeshi...4th or 3rd century BCE), who wore 2-foot-long (0.61 m) black felt conical hats with a flat brim" and gives this footnote.
This image is from Robert Hunt's 1864 survey of Cornish folklore. The conical hat was traditional in Cornwall and Wales, and it became associated with witchcraft in part because "traditional" became linked in the popular mind with "old" and old women seemed to have access to magical practices.