My sister says unicorns existed. It says so in the Bible. I asked her why they and all the dinosaurs didn't make it on the ark if they supposedly lived around the same time as humans. I mean wasn't Noah to put two of EVERY animal in the ark?! So cats and dogs survived, but no dinosaurs, unicorns or dragons??! The more I hear this story, I realize how absurd the whole thing is. And I hear smart people trying to explain away its validity.
I always though that the unicorn story in the bible was a wonderful example, of what is sometimes called the 'telephone game'. When you read the original, especially in Job, it seems obvious, to me at least, that the bible is talking about a rhino. At what time I wonder, did the misunderstanding of that become a quite different mythical creature. Or perhaps my judgment about it being a rhino is just stupid, and there really are one horned horses waiting in the woods to ravish human maidens.
Probably medieval Europe when the Bestiaries started to become popular.
I’m no expert here but it could partly be due to the discovery of Herodotus and his fantastical beasts that he tells of in ‘Histories’