That mountain range descriptor is indeed singular - the "Caballo Mountains" (Horse Mountains). The plural familiar usage, "Caballos", is a shortcut similar to calling the Rocky Mountains the Rockies. I'm quite sure that the NP map reference is indeed designating NM's Caballo Mountains. There's no verifiable provenance for the map other than NP's word. Based on posts on other subjects, I don't consider the late NP a reliable root of information, and his identifying Noss as the map's source doesn't strengthen my faith in the map either. Of course, I could be wrong.
As for the "spelling mistake", here's something rather odd from the Mariano Map (whose alleged provenance is also unverifiable), vis-a-vis those mountains: the range is labeled like so
View attachment 1741936
Looks sorta like the "incorrect Os" were changed to As, and big ones too. What's up with that? I have no idea other than possibly some sort of corrected reference to the "spelling error"? Be that as it may, the point I was making in previous posts re the H/P stone is that a significant case can be made that the horse side seems to fit better on the NM side of the Gila than the AZ side.