Pigs "first flew" in 1909 as part of a publicity stunt.
The expression "when pigs fly" is commonly used to indicate something that will never happen. However, a British aviation pioneer named John Moore-Brabazon took the saying literally. In 1909, he attached a small pig to his airplane and flew it, proving that pigs could fly, albeit with a little assistance. This was done as a joke to win a bet.
So, while the expression implies impossibility, the literal event occurred in 1909.
