No one knows exactly why the back side of a coin is called "tails," but it is easy to understand why the front side is called "heads." Common sense dictates that the reverse side would naturally bear the name of a body part located farthest from the head.
There are other theories that may explain the "tails" moniker. For example, Greek and Roman coins were adorned with heads of state on the front and animals or mythical creatures on the back. These creatures all had tails, indicating a possible link to the modern phrase. The decision regarding which explanation sounds more plausible could be made with the flip of a coin.