Rusty asks:
"Why do people trying to come up with the most outlandish, unnatural example of a fanciful creature always limit themselves to purple rhinos or pink elephants (with flying as an optional extra)?"

Because it an be easily visualized and makes the point.

And if you don't like it the invisible purple rhino, try imaging that, would like to discuss it with you man-to-man (so to speak).

DA Morgan