A visit to the doctor for a family member confirmed what I had obtained from WebMD: a ganglionic cyst at the wrist joint. Sounding frightful, such fluid-filled packets are nearly as harmless as they are mysterious. Modern medicine is at a loss to account for their appearances and vanishings, and treatment is usually no more than waiting for it to shrink; needles or surgery are options but are seldom advised since the thing could always swell and shrink again. As always, consult an actual doctor.
It may not be necessary harmful, though it has to be checked out by a doctor so that he/she can determine whether or not its harmful. It could be just a small growth which can wear off with proper treatment.