After he died, rumors circulated that weird noises could be heard coming from King John's grave. Others said they actually saw him roaming the countryside. Those stories probably aren't true, but there may be a real reason people thought the king was a shape-shifter.
At the time, and probably today, too, werewolves were seen as something evil, and that is how people saw King John. His nickname was "Bad King John," so it probably wouldn't have been too hard to convince people that he was a werewolf. Some think the rumor was just used to scare people.
So, it is very likely that King John was not a werewolf. But, werewolves themselves may have some basis in fact, even if they aren't of the shape-shifting kind. Reports of creatures that sound decidedly like them, commonly called "Dogman," come from all over, in the present day. So, who knows, maybe someday you will encounter a real "werewolf!"