The unicorn crestfish or unicornfish, Eumecichthys fiski, is a very rare, little-known species of crestfish in the...

The unicorn crestfish or unicornfish, Eumecichthys fiski, is a very rare, little-known species of crestfish in the family Lophotidae, and the only member of its genus.
The unicornfish is one of three lampriform genera known to possess an ink tube, allowing them to expel a black fluid from their cloaca as a defense against predators. The ink tube is derived from a primitive gut and runs above and parallel to the intestine.

Reference: http://www.fishbase.org/summary/3260
Image credit Sandra J. Raredon, Division of Fishes, NMNH