Ganglion. The 2 most common 'soft lumps' i encounter on the outside of the ankle are ganglion cysts-sac with fluid and fat depositis.
Get it evaluated. There is no way to make a diagnosis without an exam. The most common type of lump in this area is usually a ganglion cyst, but it can also be a lipoma or can be a more serious mass. It needs to be evaluated to determine if it's solid, soft, fluid filled, etc. Your doctor may try aspirarting (drawing fluid out) or may order tests such as ultrasound or MRI. Regardless, it needs to be evaluated.