The black swan is a species of swan from Australia. Its plumage really is black except for the flight feathers.
Before Europeans have reached Australia they thought that all swans were white. References to black swans usually implied something absurd or impossible.
Both the bird (in its Native Australia) and various European misconceptions and superstitions about it became a cornucopia of symbols that I don’t care to discuss here, especially since some became annoying after serious overuse. It is just not fair to these beautiful birds.
Black swan (Cygnus atratus) on ARKive.
Cygnus atratus on the Animal Diversity Web.
Black swan in popular culture on Wikipedia.