Young adult book; city boy spends the summer in the high country with aunt and uncle who are about to lose their ranch; crusty loner/family friend takes boy into the mountains for a few weeks; they catch and tame wild a mustang and thwart horse rustlers and save aunt and uncle’s ranch.
Stump the Bookseller
New format, same book stumper solving.
Mountain Pony, by Henry Larom maybe. There’s at least one sequel–Mountain Pony and the Pinto Colt.
One of the Fury books?
This sounds like Sand Dune Pony by Troy Nesbit , it’s back in print at the moment, so easy to find.