This is both a weathervane and an idiom. The term “when pigs fly” is common not only in English, but in Finish and a few other languages, too. You can rank the term with the same likelihood as “when grapes ripen on the willow” (Bosnian), “when chickens have teeth” (French), “when it’s snowing red snowflakes” (Hungarian), and “on St. Never’s Day” (Portuguese).
But then, here’s a flying pig. Now what?