Quote from : A Wizard of Earthsea Book

As a boy, Ogion like all boys had thought it would be a very pleasant game to take by art-magic whatever shape one liked, man or beast, tree or cloud, and so to play at a thousand beings. But as a wizard he had learned the price of the game, which is the peril of losing one's self, playing away the truth. The longer a man stays in a form not his own, the greater this peril. Every prentice-sorcerer learns the tale of the wizard Bordger of Way, who delighted in taking bear's shape, and did so more and more often until the bear grew in him and the man died away, and he became a bear, and killed his own little son in the forests, and was hunted down and slain. And no one knows how many of the dolphins that leap in the waters of the Inmost Sea were men once, wise men, who forgot their wisdom and their name in the joy of the restless sea.