Medieval royal treasuries often included "unicorn horns" thought to be an aid in detecting poison. These unicorn horns were actually the tusks of a Narwhal, a whale that lives in only the most extreme northern latitudes.
"In even earlier times, the Narwhal gave rise to the legend of the unicorn in the middle ages. During that time Narwhal tusks (usually found washed-up on northern shores) were believed to have come from a horse-like creature that had healing powers; however, according to the legend, the creature was a wild beast that could only be captured by offering a virgin as sacrificial bait."