The marine animal with a horn-like tusk that is often called the “unicorn of the sea” is the narwhal. It is a medium-sized toothed whale native to Arctic waters.
The famous tusk is usually an elongated, spiralled left canine tooth that projects through the upper lip of an adult male. It can grow to several metres long and contains millions of nerve endings, making it a highly sensitive structure. Scientists believe it helps signal social status and may play roles in competition or courtship, although its full purpose is still studied.
Narwhals are whales, not fish, and they are more closely related to belugas than to dolphins or orcas. Females usually lack a prominent tusk, though some females grow one, and a small number of males develop two. Medieval Europeans once mistook narwhal tusks for genuine unicorn horns, giving them supposed magical and medicinal powers. That belief helped make the tusks valuable trade goods long before naturalists identified the animal as an Arctic whale.