The Soldier and His Horse explained simply

A Soldier gave his Horse a plentiful supply of oats in time of war, and tended him with the utmost care, for he wished him to be strong to endure the hardships of the field, and swift to bear his master, when need arose, out of the reach of danger. But when the war was over he employed him on all sorts of drudgery, bestowing but little attention upon him, and giving him, moreover, nothing but chaff to eat. The time came when war broke out again, and the Soldier saddled and bridled his Horse, and, having put on his heavy coat of mail, mounted him to ride off and take the field. But the poor half-starved beast sank down under his weight, and said to his rider, "You will have to go into battle on foot this time. Thanks to hard work and bad food, you have turned me from a Horse into an ass; and you cannot in a moment turn me back again into a Horse."