Nickar is a demon or spirit in the Dictionnaire Infernal

Dictionnaire Infernal

The Dictionnaire Infernal is a book on demonology, organised in hellish hierarchies. It was written by Jacques Auguste Simon Collin de Plancy and first published in 1818. There were several editions of the book, but perhaps the most famous is the edition of 1863, in which sixty-nine illustrations...

. In Scandinavian, Teutonic

Teutonic Mythology

Teutonic Mythology may refer to:*Continental Germanic mythology*Jacob Grimm's Deutsche Mythologie *Viktor Rydberg's Undersökningar i germanisk mythologi I...

 and Saxon

Saxons

The Saxons were a confederation of Germanic tribes originating on the North German plain. The Saxons earliest known area of settlement is Northern Albingia, an area approximately that of modern Holstein...

 mythology, Nickar are malevolent water spirits who drown people, torment fisherman, tip over boats and throw them to the tops of shore-side trees. According to mythology, Nickars or Hnickars patrol the waters of Scandinavian fjords, where they cause tempests, hurricanes, hailstorms and blizzards. They have fish-like tails and often sit by the water combing each other's long green or blonde hair with seaweed, playing a harp or singing. Like the Siren

Siren

In Greek mythology, the Sirens were three dangerous mermaid like creatures, portrayed as seductresses who lured nearby sailors with their enchanting music and voices to shipwreck on the rocky coast of their island. Roman poets placed them on an island called Sirenum scopuli...

s, they drag to the bottom of the sea anyone who ventures too close.

Nickar are descendended from the Mermen

Merman

Mermen are mythical male equivalents of mermaids – legendary creatures who have the form of a human from the waist up and are fish-like from the waist down.-Mythology:...

 and Mermaid

Mermaid

A mermaid is a mythological aquatic creature with a female human head, arms, and torso and the tail of a fish. A male version of a mermaid is known as a "merman" and in general both males and females are known as "merfolk"...

s, or from the Nymphs of the Elbe

Elbe

The Elbe is one of the major rivers of Central Europe. It rises in the Krkonoše Mountains of the northwestern Czech Republic before traversing much of Bohemia , then Germany and flowing into the North Sea at Cuxhaven, 110 km northwest of Hamburg...

 and the Gaal, the Nix

Nix

The Neck/Nixie are shapeshifting water spirits who usually appear in human form. The spirit has appeared in the myths and legends of all Germanic peoples in Europe....

ies (Teutonic) which are kind, loving benevolent water spirits, known to rescue wayward fishermen. These lesser water divinities will sometimes leave the water to attend nearby celebrations and dance or fire festivals, where they dance and greet local fishermen.

It has been suggested that "Old Nick", one of the colloquial English names for the devil

Devil

The Devil is believed in many religions and cultures to be a powerful, supernatural entity that is the personification of evil and the enemy of God and humankind. The nature of the role varies greatly...

, is derived from Nickar, but it seems more likely to be a contraction of the personal name "Nicholas" instead.