List of idioms of improbability

There are many idioms of improbability, used to denote that something is impossible or unlikely to occur.

Flying pig (main article)
A flying pig is a symbol of an impossible event coming to pass. The popular saying "[it will happen] when pigs fly" (or when pigs have wings) is traditionally used to mean that the specified event will never occur.
Cold hell 
Rises from the general belief that hell is an extremely hot place. Expressions include "A cold day in hell", "When hell freezes over" and "A snowball's chance in hell."
If wishes were horses, beggars would ride (article)
Once in a blue moon (article)
Twelfth of Never (article on song title)

In other languages

See also