This book is at an excellent reading level for a beginning chapter book reader. It's short, with short chapters, and illustrated.
That said, I found this book difficult to get through. It sat around my house for so long that the owning library has threatened to bill me for it. It simply wasn't telling the story that was begging to be told, in my opinion, and nearly every other chapter was incidental to the plot.
I can summarize the entire plot for you (something I prefer not to do thoroughly, but I think I'm doing you a favor here).
Young Pedro lives on the plains of Argentina, an area populated by various sharecropping families. One day he catches sight of the most beautiful wild pony he has ever seen, and Juan (an adopted father to the boy) catches it for him tames it, and teaches Pedro to ride it.
One day the patrón sends word that he wants a good pony for his shiftless son upon their return, and the mayordomo has his eyes on Chúcaro (who was, after all, caught on the patrón's property). The sharecroppers figure out a solution: They will let the horse decide. If the patrón's son cannot lasso Chúcaro, he will remain with Pedro.
Of course, he can't, and is made a fool of. In a fit of anger he throws a set of bolas (think three bocce balls attached by a leather strap) at the pony's face, knocking it to the ground. When the boy approaches later, the pony gives him a good hard kick to the stomach. The boy accuses Juan of having ordered the pony to kick him, and when Juan defends himself, the patrón fires him for calling his son a liar and evicts him from the property.
Juan and Pedro ride off into the sunset forever to explore what other opportunities await elsewhere.
This is the story that took 127 pages to tell. You're welcome.