THE MOUSE-GRAY STALLION
(BOBBS MERRILL & CO [USA] 1957)
ILLUSTRATED BY WILLIAM MOYERS
There was also a German language edition
SUMMARY: Historical set wild horse story. Young Pete McCune wants to prove to his father that he is a good judge of horseflesh. But when he finds the ugly and small mouse-gray wild stallion he realises that looks alone are not what makes a good horse.
Can be found without too much trouble in the USA, although is rare elsewhere. There was also a German edition which is not too hard to find.