Alcatraz by Max Brand

Far behind him he could see the pursuers driving their horses at a killing gallop. He answered their spurt and held them safely in the distance with the very slightest of efforts. All his care was given to picking out the easiest way, and avoiding jutting rocks and sharp turns which might unsettle the rider. Just as, in those dim old days in the pasture, when the short brown legs of the boy could not encompass him enough to gain a secure grip, he used to halt gently, and turn gently, for fear of unseating the urchin. How far more cautious was his maneuvering now! Here on his back was the power which had saved him from the river. Here on his back was he whose trailing fingers had given him his first caress.

He had no power of reason in his poor blind brain to teach him the why and the wherefore. But he had that overmastering impulse which lives in every gentle-blooded horse -- the great desire to serve. A mustang would have been incapable of such a thing, but in Alcatraz flowed the pure strain of the thoroughbred, tracing back to the old desert stock where the horse lives in the tent of his master, the most cherished member of the family. There was in him dim knowledge of events through which he himself had never passed. By the very lines of his blood there was bred in him a need for human affection and human care, just as there was bred in him the keen heart of the racer. And now he knew to the full that exquisite delight of service with the very life of a helpless man given into his keeping.