Nuckelavee (RWBY)

The Nuckelavee Grimm (unofficial) is a large, horse-like Grimm and a major antagonist in Volume 4 of RWBY. It attacked multiple towns in Anima and is responsible for the death of Lie Ren's parents.

Appearance
The Nuckelavee Grimm is a horse-like creature with an equine main body and a skinless humanoid creature that is fused with it. Its forelegs are shaped like claws and it has hooved hindlegs that leave a characteristic hoofprint. The humanoid body has long curved horns on its head, a mouth that looks almost sewn shut, and two long elastic arms each with two clawed fingers. On the back of the human torso are boney spikes along its spine and a variety of weapons from previous battles stuck in it.

When walking on its equine legs, its attached humanoid portion hangs limp to one side, making its arms drag along the ground. Once the equine portion stops to stand still, only then does the humanoid portion of the creature seem to "come to life", doing so with a spasmodic twitching movement. When enraged, the spikes on its spine lengthen, as do its horns, its pupils become vertical slits, and its mouth opens completely.

Background
According to Ozpin, the Nuckelavee Grimm was created by the God of Darkness. Centuries later, the creature attacked the town of Kuroyuri, destroying it beyond repair and killing most of the citizens living there; including Lie Ren's parents.

Volume 4
In "Remembrance", Team RNJR comes across the ruined town of Shion, which has been attacked and destroyed by the Nuckelavee Grimm. Ren and Nora notice the creature's footprint in the dirt, but decide to say nothing.

In "Two Steps Forward, Two Steps Back", Ruby and Jaune take Qrow down the path to the ruined town of Kuroyuri, unaware of the Nuckelavee Grimm's footprint beneath them.

In "Kuroyuri", Ren and Nora discover a cave where the Nuckelavee Grimm has been living and leaving behind tokens of its conquests. Upon looking out the other side, they see the trees ruffling as the Nuckelavee Grimm roars and makes its way toward Kuroyuri.

In "Taking Control", the Nuckelavee Grimm arrives at Kuroyuri, roaring as it spots Team RNJR.

In "No Safe Haven", the Nuckelavee Grimm battles Team RNJR, overpowering them with ease. Ren attempts to attack it and gets pinned by the Grimm, forcing Nora to save him but getting beaten and depleted of Aura in the process. Ren breaks free and charges the creature again in a blind rage, but is also defeated and depleted of Aura. Ren soon comes to his senses after some words from Nora and together Team RNJR devises a plan to defeat the Grimm; pinning the humanoid portions arms to the ground while Nora kills the equine portion with Magnhild. Ren then severs the humanoid portions limbs before decapitating it, killing the Grimm and avenging his parents as well as everyone the creature has slain.

Abilities and Powers
The Nuckelavee Grimm is quite powerful compared to the rest of its kind, being responsible for the destruction of both Kuroyuri and Shion. Its humanoid portion has extendable arms and a discomforting screech that briefly stuns its enemies while the equine portion attacks with its legs and hooves. When it gets angered, the Nuckelavee Grimm increases its strength, giving it longer spikes and causing the humanoid portions mouth to open completely.

Trivia

 * The Grimm appears to be based on the Nuckelavee, a creature from Orcadian mythology with origins from both Norse & Scottish mythology. The Nuckelavee was commonly blamed for disease and destruction of crops.

Source
Nuckelavee Grimm on the RWBY Wiki.