Why is tails called tails on money?

Per Dictionary.com, the term can be traced at least as far back as the 17th century and probably much earlier. That means it's not American and not related to buffalo nickels. Many European coins carried a portrait of the reigning monarch on one side, and this became known as the "head" side of the coin. The back side, being the opposite of the head, naturally was referred to as the part of the body opposite to the head for most animals - though not humans, interestingly enough.