Sun Horse, Moon Horse by Rosemary Sutcliff

From the moment he is born, Lubrin Dhu is different and his unusual talent for drawing places him even further apart. So when his tribe is conquered and Lubrin is appointed its mouthpiece, he is treated with the utmost suspicion. What is the bargain that Lubrin has struck with the enemy lord? And why does he make a horse - a huge horse, high up on the hillside, cut out of the chalk? How can this set his people free?