Shadow lives on his uncle and aunt's ranch where he helps train horses for polo. He sets his heart on catching the beautiful wild roan stallion he has named The Runner.
Set on a Wyoming ranch...featuring a wild mustang...gotta be the very last horse book I would expect to talk about training polo ponies.
But that is indeed what Uncle Nathan, a horse dealer by trade, is mainly using his ranch for. With the addition of a shamrock shaped training track, he has a small stable of prospects he's training in the sport to increase their value (apparently even in the 1950s, top-tier polo ponies could go for 5 figures!), and Shadow spends the early part of the book helping train and exercise them along with a hired hand (a former jockey), and Nathan eventually hires an experienced trainer to polish their training to perfection.
However, the majority of this book is about his nephew Shadow's efforts in capturing and taming a particular lightning-fast roan mustang for his own. True, he does have hopes that he can turn this horse into a jewel of a polo prospect, but I don't think it's too much of a spoiler to say that he never actually gets around to that kind of training, as his focus is kept busy not only getting the horse to trust him and take to the saddle, but stop breaking out of his corral and taking flight every time he gets spooked.
Other elements of this book provide plenty of adventure, from unscrupulous mustang hunters to multiple wild killers (a grizzly and the VERY incongruous idea of a...). There are also chapters devoted to the horse's point of view when he is in the wild (and/or temporarily escaped back to it), including a significant segment near the end when . ---------------- Bottom line: I really liked this one. It's a very solid 50s ranch/horse/boys' adventure story, though I did spin myself into a tizzy for about an hour trying to figure out if this book was a sequel, because there's so much casual info-dumping about the recent past that it feels like they're just trying to jog your memory (but as far as I could find with limited info, despite the authors' prolific output it's a standalone).
P.S. I have never been so disappointed to only own a cheap edition of a book because not only is this missing the original illustrations, apparently the first edition has gorgeous endpapers detailing a color illustration of the ranch! Not to mention a nicer cover illustration -- it's the same base image but drawn in a considerably more skilled hand. Not on Goodreads yet unfortunately.
I read this as a child and loved it so much that I've kept it all these years... over 50!! I'm going to reread it just to take a trip down memory lane and then encourage my Grandsons to read it also.