The narwhal, also called the rhoar or sea-unicorn, in Latin monoceros, is a fish of the northern seas, which some reckon to be a sort of a whale. It bears on its nose a long spiral horn, which is uses to pierce ice. The name comes from Icelandic náhvalur and means "corpse whale" (nár – corpse, hvalur – whale), which probably alludes to the animal's coloration.
As says Thomas of Cantimpré, with Olaus Magnus and Conrad Gessner repeating, the monoceros is a marine monster that carries on its forehead a single horn. This horn is so large and so sharp that it can easily pierce the hulls of ships, sinking them and killing their crews. But thankfully, as fierce this fish is, it is also very slow, so sailors may spot it from afar and avoid its wrath.