The 1 Ring

The 1 Ring can only be found by fishing, referencing the discovery of the One Ring by the hobbits Déagol and Sméagol while on a fishing trip (in Tolkien's Lord of the Rings).

Source
This item can be found by fishing in any low level fishing area, such as the canals in Stormwind and other fishing areas in Horde and Alliance main cities and secondary areas. However, it has an extremely low drop rate and even an avid fisherman is unlikely to ever see one.

The 1 Ring binds on equipping, so can be sold, and does appear in the Auction House from time to time.