#004 for 2020
The Narwhal (Monodon Monoceros), as suggested by @foodandpuns, are often referred to as the “Unicorns of the sea,” but its characteristic horn actually grows out from from a protruding canine tooth.
Interestingly, it is mostly the male of the species grows an elongated tusk. In some very rare circumstances, both canine teeth can grow into tusks resulting, of course, in a two-horned individual.
Narwhals make their home in the Arctic seas between Iceland, Canada & Russia and are a close relative of the Beluga Whale, in the Monodontidae family.
Yet scientists still do not know why or how the horn came about and how they make use of it. It is a wonderful natural mystery.
Narwhals are currently classified as Near Threatened.