People use the term “when pigs fly” to state how unlikely something is to happen. It’s believed the phrase originated in Scotland or Germany.
The saying appears in Lewis Carroll’s Alice’s Adventures in Wonderland.
"I've a right to think," said Alice sharply... "Just about as much right," said the Duchess, "as pigs have to fly."
Pigs don’t have wings but, on Nov. 4, 1909, John Moore-Brabazon, an English pilot, brought a pig with him on a flight.