The modern idea of werewolves seems to have arisen somewhere in the Middle Ages, after the spread of Christianity. But I vaguely remember a story in the Satyricon (specifically at Trimalchio's dinner) about a man turning into a wolf at the full moon.
Are there other stories of people transforming into wolves from Classical or pre-Classical times? (And back again: while Ovid writes about a lot of transformations, they're generally permanent.)