The definition of the SOCK MONSTER according to the Urban Dictionary:
Found in (or around) washing machines and tumble driers
Thought to be genetically modified, highly evolved organism. Although never actually sighted evidence points to its habits and breeding cycle. A voracious predator the sock monster preys on single socks always leaving behind one of the pair it has captured. Mystery surrounds this behaviour although zoologists surmise this may be an instinctual mechanism for long term survival. Able to cross great distances at speed and unseen there is almost no known method of defence.
Currently there is a research program to capture one and study its life cycle to provide some relief. The only known fact is sock monster numbers are proportional the number of people in residence at one location.