I once laughed when a child with chicken pox pointed out to me a single spot, calling it a "pock". I had been under the impression that they had mistakenly thought the word "pox" was plural, and so could be singularised.
Turns out I was wrong. "Pock" is a perfectly valid word.
That got me thinking about which word came first. Was the word "pock" invented, based on the existing word "pox"? Or did "pock" always exist, and "pox" is merely the shortened version of the plural "pocks"?