In the First Age of Middle Earth, Sauron assumes the shape of a wolf to fight with the good hound Huan. Huan takes wolf-Sauron by the throat and will not let go, so Sauron shifts into a serpent, then into a monster, and finally into his accustomed form in an attempt to break loose, but cannot. Eventually Sauron negotiates for his release, and upon securing it, assumes the shape of a vampire and flies away. The most notorious shapeshifter, of course, has been the devil. This was taken as common knowledge in the Middle Ages.
Stories abound of the devil's numerous metamorphoses.
He frequently appeared as an old man or woman, an attractive boy or girl, a pauper, fisherman, merchant, student, or shoemaker. He could appear holy, posing as a priest or pilgrim. He was often persuasive, posturing as a mathematician, physician, or grammarian. He could appear as a natural wonder such as a whirlwind. He shifted his shape to suit his purpose.