Nuckelavee

A Nuckelavee (noo-keh-LAA-vee; Sc.Gael. Nuckelavee) is a horse-like Wesen that first appears in. One was sent by the Royal Families to get Nick's Key.

Biology
Their appearance resembles that of a skinless horse's head with black blood pulsing through transparent veins, and muscles and sinews visible. When woged, they have a long mane of hair, horse-like teeth, grayed skin, and hooves.

Nuckelavee are a physically strong Wesen species and able to hold their own against a Grimm, however Nick Burkhardt was able to defeat one in melee combat (albeit using a maul). Nuckelavee are also a quick and stealthy species. When attacking in their Wesen forms Nuckelavee use their hoof-like hands (all the fingers with the exception of the thumbs) as blunt weapons, which combined with their impressive strength are at least capable of leaving large dents in the outer hull of car trailers.

Behavior
Nuckelavee are stealthy, quiet, and calm Wesen, who approach tasks logically. They have no qualms with stealing or murder, as long as it helps them to succeed in their appointed task. As such, they commonly find employment working for the Seven Royal Households. They also possess incredible determination, and would rather die than fail their tasks.

Compared to Hundjäger and Mauvais Dentes, the Nuckelavee seems much more honorable or in that it only goes after the intended target, and leaves all bystanders unharmed, as seen when David Esquibel left Juliette alone after realizing that the key wasn't in her house. However, this may be due to the fact that it appeared to try to be stealthy and remain unnoticed by Nick or anyone else.

Captain Renard receives a call from his confidant warning him that a Nuckelavee is on the way to Portland from Rome via JFK to get the key. Renard then assigns Franco to find out who this person is.

Franco reports back that three people were traveling from Rome using the specified itinerary, two are accounted for but one is missing. He also reports that a cab and its driver went missing 30 minutes after the plane landed and have not been heard from since. He puts an APB out on the cab. Captain Renard warns Nick and Hank about a fugitive in town who has an issue with cops. The only description he has is that the person is a Caucasian male.

David is watching the Medical Examiner's Office and when Nick and Hank leave, he follows them in his stolen cab after he woges.

David is watching when Nick and Hank leave the Stanton residence on the way to the park.

David is on the roof of the Spice Shop watching through the roof window while Nick, Hank, and Monroe cure Rosalee and Ryan Gilko of Fluvus Pestilentia.

The Nuckelavee enters the bar where Bud is telling Nick about his meeting with Juliette. Nick senses something but David has moved out of sight.

Captain Renard receives a call from his French contact, telling him that he has found the identity of the Nuckelavee and has sent the file. The file contains David's name, alias and picture.

The Nuckelavee breaks into Nick and Juliette's house, and rummages through the drawers to look for Nick's key, using a rough paper sketch he has as a reference. Juliette comes home halfway through his search and he is forced to slip back out of the house.

Later, the Nuckelavee attacks Nick while he is leaving the trailer after studying Seelenguter. After a brief struggle outside and inside the trailer, Nick eventually manages to get a maul from the weapon cabinet and kills the Nuckelavee, but not until Nick has suffered some close escapes. He finds the picture of his key in the dead Nuckelavee's jacket and displays a grim smile. The next day, Wu reports that his body was found floating down the river, and it looked like someone had attacked him with a hammer.

Trivia

 * It appears to be based off the Nuckelavee in Orcadian folklore.
 * It is the first Wesen to have its name taken from the Scottish Gaelic language.