The first descriptions of a unicorn were traveller's tales of Africa where there were wild "horses", with a horn in the middle of the forehead. Rhinos!
These stories were told amongst many traders, and Viking traders (Norsemen) had a great idea. Sell the "tusks" of the Narwal as Unicorn horns! It went on for years; always adding to the myth.
Queen Elizabeth I, once paid a fortune for one of these "Unicorn horns", to a Norwegian fisherman.