Black Horses for the King — Anne McCaffrey

Lord Artos later to rule as the legendary King Arthur knew he could defeat the Saxon invaders if only he could find a race of horses swift and strong enough to carry warriors in full regalia fast and far. And so he set out for the Continent, in search of the famed horses of the desert.

The key to Lord Artos's plan was the young runaway Galwyn Varianus, whose gift for horsetrading was second only to his skill with horses. What no one expected was how crucial Galwyn would be to the upcoming battles as he mastered the secrets of the iron shoes that would protect the desert horses' delicate hooves...

This fast-moving historical fantasy -- the story of King Arthur us it has never been told before--is about the beginnings of the British cavalry, as recounted by a boy growing up in exciting a perilous times.