Buffalo Medicine by Don Coldsmith, (Spanish Bit Saga Series, Book 3) from Books In Motion.com

When they mastered the ways of the Elk Dog, the horse, the band prospered. Yet they knew the old traditional ways must not die. Owl's father had changed the lives of The People bringing the horse from his native Spain. Now the younger son of the legendary chief must make his own reputation by learning the way of the buffalo from an ancient medicine man. He pursues his own vision quest, a perilous journey taking him through savage lands where he is captured by his enemies and enslaved. Working in the Spanish mines he wins the heart of a brave and beautiful woman, and his freedom. Awaiting Owl is honor among The People--and a deadly rival who claims that honor as his own.