The unholy union between a pirate and a ninja. Pirates and ninjas are widely known to be mortal enemies and are locked in eternal oppisition to each other.
Pirate-ninjas rarely exist in nature because they have extremely short half-lives; upon their creation they rapidly decay in a violent burst of short-wave radiation, heat, shurikens, and parrot feathers.
Pirates vs. ninjas - that ages-old question. Who would win?