Obviously, the man had some connection with the supernatural. But what exactly was/is he? A vampire? Zombie? Alien? None. He is a wizard, and a Metamorphmagus, like Tonks. He looks and sounds odd simply because that's normal for the wizarding world. His shape-shifting abilities would explain (a) how he is able to change his hair and weight so radically from one movie to the next, and (b) how he is nowadays able to appear on TV in the form of a maggot, hanging lamp, fish, Simpson's character, Tweedy Bird, the Genie, etc.