The dreamseeker trilogy - Jenny Oldfield

Silver Cloud, a wild horse who is noble, brave, fast and strong, roams the plains. He belongs to a society which is more sophisticated and peaceful than that of the neighbouring human tribe which now, on the verge of extinction, needs his help. Silver Cloud's ruling spirit, Ghost Horse, sets him the task of saving them and, as Red Hawk, the leader of the tribe, is old and dying, his young grandson, Four Winds, is sent to accompany him.

Silver Cloud and Four Winds must complete three tasks which will redeem the tribe, but during their journey they must battle against evil animal spirits as well as face the storms, fires and earthquakes that threaten their quest.