I recently obtained a Silver Angel collectable coin, where the back side bears an image of an angel fighting a dragon: ![silver angel coin](https://i.stack.imgur.com/vuIvO.jpg)

I sort of realized, as I was looking at it, that for probably the first time in my life, I was holding a coin where the image on the back *actually portrays a creature with a tail prominently displayed.*

Everyone's heard of "heads or tails?", the traditional invocation for a coin toss. The head is obvious: most government-issued coins, from antiquity to modern times, have borne the bust of some famous ruler on one side. But most coins do not come with some tailed animal on the back, so where does the ubiquitous expression come from?