Numerous theories have been presented over the centuries to explain the function or purpose of the narwhal tusk, the strangest tooth in nature: as a spear for hunting or a tool for digging, as a weapon of defense or aggression, an instrument for breaking ice or sound propagation, or even as a swimming rudder or an organ for breathing.

Perhaps Narwhals are the evolutionary descendent of unicorns?

In all seriousness, it would be cool to have a horn.