A rubber duck (or ducky) was a type of children's toy on Earth, in the form of a Terran duck. Made of rubber or similar material, it was most commonly used by very young children while bathing.
Commander Nyota Uhura of the USS Intrepid II, during a confrontion with Romulan Commander Avrak, insulted him by comparing his ship, the IRW Adamant, to a rubber ducky. (TOS novel: Vulcan's Forge)
In 2341, Data was given a squeaking rubber ducky as a birthday present. (TNG novel: Deceptions)