Black swan
A swan with dark feathers — a rare event that breaks what the count had taught you to expect.
A black swan in a dream is the dream's image of an event that breaks the count. For centuries the European mind held swans to be white by definition; one black bird at the edge of the lake quietly invalidated the rule. The dream brings the black swan when something in your life — a person's behaviour, a piece of news, an outcome the model said could not happen — has arrived and is forcing the model to be rewritten. A black swan seen and accepted is the dream affirming that you are letting the data correct the theory. A black swan refused, with the lake claimed to be still all white, is the dream noting that you saw the exception and are choosing to keep the broken rule.