What animal was called the unicorn of the sea?

It is called a Narwhal, which is a medium sized toothed whale, that lives in the Arctic. The narwhal males are distinguished by a characteristic long, straight, tusk extending from their upper left jaw. Found primarily in Arctic and Greenland waters, the narwhal is a uniquely specialized Arctic predator. In the winter, it feeds on mostly flatfish at depths of up to 1500m under dense pack ice.