A fox's tail has many uses, including allowing it to communicate with other foxes

Animals are always making up for their inability to speak to each other in incredibly interesting and amazing ways. The fox's tail doesn't just help it balance, thanks to its thickness, but it also keeps it warm in the winter and can be used to signal other nearby foxes in various ways. Another way foxes communicate is to "mark their territory" by urinating on nearby landmarks, very similar to other animals that do the same.