The way renaming (or just naming) works is extremely counterintuitive. I could not name it anything I chose when I created it. Later, I can find it in Preferences, and then I can rename it, but I still can’t put it at the top of my list of libraries (even if I add a “_” at the beginning of the name).
Hmm, New Library does seem to enforce the .pretty. Normally I create the directory from outside KiCad from past habit. There is nothing special about the New Library action, it’s just a mkdir followed by a register to fp-lib-table.
An underscore will place your library at the bottom of the list. This FAQ has a blow by blow account of setting up personal libraries and naming them so they will appear wherever you wish in the Schematic and PCB editor lists.