From: DICTIONARY OF AMERICAN NAVAL FIGHTING SHIPS, Vol. VII, pp. 405-06.
The narwhal-a marine mammal which inhabits Arctic seas and is related to the whale and the porpoise-has a single tusk which often grows to nine feet in length suggesting the horn of a unicorn. Indeed, the narwhal is frequently called the sea unicorn.
The contract for the construction of Unicorn (SS-429)-a Tench-class submarine authorized on 9 July 1942 and scheduled to be built at Philadelphia, Pa., by the Cramp Shipbuilding Co.-was cancelled on 29 July 1944.