First: I hope you stick with "Gringo." "Pistolero" is a bit long for a dog's name. I deduce it would inevitably be shortened to "Pisser," or something equally unflattering.
I hope your inspiration, listening on your iPod, was either the Allen Wayne Damron OR Tim Henderson version of "The Gringo Pistolero." I have ALWAYS been a big fan of the song, and the story behind it. I heard the back story directly from composer Tim Henderson, which makes it all the more valuable to me.
Tim based the song/story line loosely on Army Captain Leonard Matlack's rescue of the two Army aviators from Mexifcan
insurgentes in 1920. Hey, the excellent song is about as authentic as the story line in "The Wind and the Lion" movie - - Mrs. Edith Pedicaris was in reality a male diplomat . . . .
Anyhow, if for some reason you decide that "Gringo" is not exactly right, I submit that "Leonard" would make a good
nombre del perro.
The border bandit troubles are referenced in the review of the book,
Bosque Bonito here. The entire Captain Matlack/Chico Cano interaction is detailed in Elton Miles' excellent book,
More Tales of the Big Bend, Texas A&M Univ Press, 1988.
I hope to meet Gringo, Leonard, whatever, in the future.
Best
JPG