Anne McCaffrey 
Black Horses For The King 

Lord Artos is on a historic quest – to search out the finest black horses ever known to man. He has vowed to drive the invading Saxons out of Britain, and he needs big, strong, powerful horses to carry his army into battle. Galwyn, a young Roman-Celt, leaps at the chance to accompany the group, little guessing the adventure ahead of him – or the price he will have to pay for his loyalty…