Which sea animal has a horn like a unicorn?

The narwhal, and the horn is a tooth - the left canine of the upper jaw, which in males grows straight out through the lip in a left-handed spiral to three metres. Narwhals have no other teeth to speak of, and do not use it for chewing.

It is also not really a weapon. The tusk is porous, with millions of nerve endings running to the surface, which makes it a sensory organ - it can apparently detect changes in salinity and temperature, and drone footage in 2017 showed narwhals using it to stun cod with quick sideways strikes. Males do rub tusks together, which may be display or may be maintenance.

Medieval Europe knew the tusks without knowing the animal. Vikings and later traders sold them as unicorn horn, at prices many times their weight in gold, credited with detecting and neutralising poison - Elizabeth I is said to have received one valued at the cost of a castle. The trade worked precisely because the source was a secret, kept for centuries by the people who controlled the Arctic hunting grounds.