I am looking for animal-based alternate expressions to the 'black swan' for a rare event (or a rare item, even though that's not precisely what black swan signifies).
I believe I have heard other (and possibly more humorous) animals mentioned in this kind of 'rarity' context, but can't recall which.
Slang/idioms/colloquialisms ok, some humor preferred (in contrast to 'black swan' connotations).
Best Answer
One obvious (thanks to Moby-Dick) term for something rare and elusive is white whale—although the albino form of any animal (except laboratory rats and mice) that is normally colored can be used as a metaphor for rareness. For a discussion of rare albino whales (as opposed to mostly white beluga whales) see National Geographic's rather paradoxically title article "How Did Rare White Whale Spotted Off Australia Get That Way?"
Another animal with a famously rare color phase is the blue lobster.