Horse & Pony Books:
(WORLD [USA] 1957)
ILLUSTRATED BY ROBERT FRANKENBERG
SUMMARY: Historical horse and mule story. When Ward's mule Cyclone forms an attachment to Captain Marcy's horse Bluebonnet, they are allowed to follow them and work on the wagon train as it heads west.
LEOPARD HORSE CANYON: THE STORY OF THE LOST APPALOOSAS
(WORLD [USA] 1959)
SUMMARY: Ted finds a spotted Appaloosa colt in with the cattle and wonders where it has come from.
(WORLD [USA] 1969)
ILLUSTRATED BY SAM SAVITT
SUMMARY: Cowboy type story, not sure how much horse content.
Ride Gaucho is by far the easiest of these three to find and is probably more collectable due to the Sam Savitt illustrations. Of the other two, neither are common, with Cyclone being pretty rare. However they are not usually that expensive if copies can be found. All three are hard to find outside of the USA.