Unlike mythical unicorns, the narwhal (Monodon monoceros) is real and most closely related to beluga whales (Delphinapterus leucas)! However, that is not a horn on their head but a tooth that males (and some females) have. The functions of the narwhal tooth are still being researched, but it is suggested that it can be used for sensing salinity and water temperatures through ice floes and channels, as well as determining dominance hierarchies in males (through displays, not weapons).
Sketch and fact contributed by Karli Chudeau
Heide-Jørgensen, M.P. (2009). N – Narwhal: Monodon monoceros. In W. F. Perrin, B. Würsig, & J. G. M. Thewissen (Eds.), Encyclopedia of marine mammals (2nd ed., pp. 754-758). San Diego: Academic Press.