alt Hacker NewsBecause 100,000 years is rather extreme to have any kind of myth survive. Instead the ultra long spiral horn likely comes from narwhal as in people could hold and sell “unicorn” horns.
https://www.thevintagenews.com/2017/01/23/in-medieval-europe...
Accounts of unicorns in antiquity had rather different horns.
Woolly rhinoceroses (related to the Sumatran rhinoceros and a different species from the one from the link in the posting above) have continued to live in Europe and Asia until much more recently, i.e. until around ten thousand years ago (i.e. around the same time when humans were forced to switch from hunting to eating seeds, presumably because of the depletion of the big animals that made hunting profitable).
That is certainly recent enough for their memory to persist in myths.
As you say, the narwhal tooth is indeed the source used for most medieval illustrations of unicorns, but not the source of the legends about them.