The mutant has developed a special tail. The tail is not necessarily a balancing tool, but more of an “additional limb”.
The tail can grapple objects like a hand (much like a new world monkey’s tail), allowing the mutant to hang from tree limbs, for instance, leaving the arms and legs free to perform other actions of a more complex and agile nature. Some typical bonuses granted by the mutation might be a +2 bonus to Dexterity checks when climbing or balancing (+4 if climbing trees), etc. The tail can also be used in more mundane ways, such as holding a light source, so the character can employ both hands to fight. The tail cannot use weapons or hold a shield, however, as it is not quite that dexterous.