Of the many mysteries of language, conventional nicknames rank somewhere on the “intriguing” to “hunh, who’d a thunk it?” Spectrum.
John —> Jack isn’t plain and straightforward, like James—> Jim or even William—> Bill (forget Will, it’s not even a nickname it’s shorthand).
Others puzzle me, especially ones like Margaret—>Peggy. (Disclosure: this one doesn’t puzzle me anymore, I looked it up 50+ years ago just before I married a “Peggy.”)
But of all the odd transformations you can think of (and please reply with them), the one I have never seen a satisfactory explanation for is Francisco—>Pancho.
And we must of course exclude Pancho Barnes, since we know her given name wasn’t Francisco. Then again, how DID she get to be Pancho? (Don knows who I mean, the rest might need to look her up, which I encourage – fascinating story)