Bewitched (1964) : 8x3

Sam and Darrin visit Inverness, Scotland, the place famous for the Loch Ness and its legendary monster. It seems that the monster is actually a warlock named Bruce who was transformed by Serena. To save Bruce's life, Sam calls on Serena to get him out of the spell. Once transformed, Bruce decides to get even with Serena by giving her a mermaid's tail! The reporters go crazy about the Loch Ness Monster, and Bruce wants to be a monster again. So he changes Serena back and Serena returns the favor by sending him back to the way he used to be, but with a little understanding about how important he really is.