The silver brumby.

A silver brumby is special, but he will be hunted by man and horse alike, and must be stronger than both. Thowra, the magnificent silver stallion, is king of the brumbies. But he must defend his herd from the mighty horse, The Brolga, in the most savage of struggles. But that is not the only danger. Thowra needs all his speed and cunning to save his herd from capture by man. In a desperate chase through the mountains, it seems there is no longer anywhere for him to run to...

Blurb from the 1968 edition published by Granada (first published 1958)