When pigs fly is an idiom used to express that something is unlikely to ever happen, in the same way as the fact that pigs cannot fly at all.
1. Pigs would fly if man was able to live on Jupiter!
2. Satish's mother assured him there were no monsters under his bed and it was like saying that pigs could fly.
3. Mohini's friends refused to believe that she was capable of taking up the responsibility of organizing the entire festival by herself as she was the bumbling type; even pigs could fly, but Mohini could not get the simplest task done properly.