The story starts with this cartoon, sent to me by Don Steiny for pretty obvious reasons:
The cartoon is one of several offering, as an explanation for why unicorns went extinct, that Noah took two male unicorns onto the ark, so of course they couldn’t re-populate the species after the Flood. (In another version, the unicorns are explaining Noah’s mistake to him.) In another vein of cartoons, the extinction of unicorns is explained by their having missed the sailing of the ark, through a misunderstanding about the day or the time of day of the ark’s departure.
Side matters: one, the source of #1, which is obviously a professionally drawn cartoon; and two, the rise of a rainbow – butterfly – unicorn association in popular culture (a modern wrinkle in unicornology).