In case flying pigs don’t seem unlikely enough for your tastes, you can now add this bad-boy to your repertoire:
Cuando las loras escupan. – When parrots spit.
Turns out that these sorts of hyperbolic expressions of impossibility are an incredibly universal phenomenon across cultures and languages. They even have a fancy Greek name: adynata. Click here to enjoy a list of adynata in 20 different language!