What is the difference between a Black Swan, a Flying Green Monkey, and a Unicorn?
First off, a Black Swan is an unexpected event that is rationalized to be obvious in retrospect. It's based on theory elaborated in a book by Nassim Nicholas Taleb. The most important distinction that Black Swans have apart from Flying Green Monkeys and Unicorns is that Black Swans actually exist whereas FGMs and Unicorns reasonably cannot.
The term Flying Green Monkeys refer to a pessimists fantasy about a highly improbable event or situation that might happen. Often they are paranoid delusions based on partial or incorrect facts and the actual probability that something may happen. They are related to Black Swans in that they live in the land of chaos and the unexpected (extremestan in Taleb's terms) but they are distinctly different because they are seen as a negative catastrophic event that can be planned for. To use a traditional statistical model, these are things that are soo far out on the end of the bell curve that they are reasonably impossible (like a 50ft tall person... or a "Wizard of Oz" style flying green monkey).
Unicorns, on the other hand, are the optimists fantasy and the flip-side of a Flying Green Monkey. Unicorns are a fabrication based on highly improbable events or situations that have a positive outcome. Good examples of Unicorns are the "inevitable payout" founders see when they begin working on a startup, or the gambler who "knows" he's going to hit it big on the next roll of the dice.
The biggest difference between these three things are in how people react to and perceive them. Black Swans can only be seen in retrospect and are highly sensitive to your point of view and initial conditions. They may SEEM just as impossible as a FGM or a Unicorn from a particular perspective, but to other observers in a different location (in time or space) they seem obvious or even trivial. More importantly, they are never predicted ahead of time. You cannot plan for Black Swans ahead of time, but simply react to them after the fact.
FGMs and Unicorns, on the other hand are people's reactions to chaos and their attempt to craft a belief system that will accommodate the chaotic. Engineers often run around hunting for Flying Green Monkeys to ensure their systems are robust. Serial entrepreneurs are always tracking unicorns and are CERTAIN this new set of tracks must lead to a veritable DEN of them.
To be successful, I think it is prudent to not focus too much attention ahead of time about finding Unicorns and avoiding Flying Green Monkeys, but be able to identify and react to Black Swans and exploit these situations quickly and effectively.