A fish having a single hornlike process on the snout, as a swordfish. Also: the narwhal, Monodon monoceros. Now chiefly historical.

The Narwhal.

A small Arctic whale, Monodon monoceros (family Monodontidae), which has only two teeth, located in the upper jaw, one (or sometimes both) of which…

= sense 8b.

By extension apparently applied to the narwhal.

The narwhal or sea-unicorn, Monodon monoceros.

As the name of a fish, shell, etc., having a projecting horn or horn-like process, or regarded as… The narwhal or sea-unicorn.

Originally: the narwhal, Monodon monoceros. Now (in form Monodon): the monotypic genus (family Monodontidae) that comprises the narwhal.

= sense 8b.

= unicorn, n. 8b.

= sea-unicorn, n. 1.