You grow a tail that can grasp and hold objects.
Benefit
A prehensile tail grants a +2 mutation bonus on Acrobatics checks made while balancing. It can also grasp and manipulate a simple object that the creature could easily hold in one hand. A prehensile tail cannot be used to operate a piece of equipment that requires opposable digits or fine motor control.
A creature can "hang" from its prehensile tail indefinitely by wrapping it around a larger object, thereby freeing up its other limbs. The prehensile tail isn't dexterous or strong enough to fire ranged weapons or make melee attacks, however.