black swan

noun3

1. •
   Cygnus atratus, a swan with black plumage and a red bill which is endemic to Australia.

   • If you tell me that there is or that there is not a black swan or a white raven, I want to know where you mean; and that you can only show me,—no description will answer the purpose. — 1884
2. •
   (figurative) Something believed impossible or not to exist, especially if an example is subsequently found; also, something extremely rare; a rara avis.

   • I do but speake in the person of Demetrius, & vnder Hippolita shadovv vvhat I intend to the rare, and neuer enough vvondred at Mospa, the black svvan of beauty, & madg-hovvled of admiration. — 1605
3. •
   [before a noun] (figurative) A rare and hard-to-predict event with major consequences.

   • Worried that Greece could go belly up? So-called black swan funds – named for rare and unexpected events – offer a way to profit in the event of a market collapse. — 2011