Southern Narwhal by @becca (Sykotic Orka)

In 1615 a ship was struck by a horned animal in the mid Atlantic, the horn itself found embedded in the hull; in 1620 a small, dark blue porpoise-like animal with a high fin and a tusk 1-2 feet in length was sighted swimming in the Southern Atlantic off South Africa, while in 1893 a narwhal-like animal was seen in the Bransfield Strait off the Antarctic Peninsula.

Narwhals are a strictly northern species and that the Southern Narwhal was either a mis-identification of a pre-existing species or populations of Narwhals or Narwhal-like animals probably thrived around Antarctica at some point or other. One other probable creature that had been suggested was a sword fish (dark blue, high fin with the long bill that could easily be mistaken as a horn) if it weren't for the fact that they are a tropical species.

Reference: Mysterious Creatures: A Guide to Cryptozoology by George M. Eberhart (2002)