I found this old addon for a very simple Pet XP bar, modified it for Classic, and everything works except the actual variable length bar that shows the XP progress.
I have attached the addon. It is the PetExpBar defined on lines 82-94 and updated by the function at lines 112-116 that doesn't work (I've examined the frame and the width is being updated, but nothing shows on the screen).
I'm sure it's something stupid. Could someone point me in the right direction, please?