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Narwhal
An arctic cetacean (Monodon monocerous), about twenty feet long. The male usually has one long, twisted, pointed canine tooth, or tusk projecting forward from the upper jaw like a horn, whence it is called also sea unicorn, unicorn fish, and unicorn whale. Sometimes two horns are developed, side by side.
Open English WordNet places these words in at least one shared synset. They share a lexicalised sense, but context, register and collocation should still be checked before substitution.