He is a skin-changer. He changes his skin: sometimes he is a huge black bear, sometimes he is a great strong black-haired man with huge arms and a great beard. I cannot tell you much more, though that ought to be enough. Some say that he is a bear descended from the great and ancient bears of the mountains that lived there before the giants came.
It is curious as to why Beorn and the skin-changers are never seen or heard from again in later works by Tolkien. He remains a mysterious character, much like Tom Bombadil in LotR.
