The Nuckelavee Monster, much like its namesake, is a quadrapedal Gem monster with four, horse-like legs attached to a red body, the color resembling raw meat. It stands at nine feet tall and seven feet long, its body similar to a horse and rider. A humanoid-like torso, arms, and head is attached to the back, with very long, thin arms, and reptilian-like eyes. Two gemstones are located on both heads.
The Nuckelavee Monster was one of the Gem experiments presumably worked on within the Tunguska Gem Facility while it was still active, the main subject being fusion. Both Gems involved in the fusion became corrupted, resulting in the monster being fused amongst the two. Ever since then, it roamed within the Tunguska Gem Facility.
It was found thousands of years later by the Ruby Squad, and the Crystal Gems' Ruby, as its activity began to catch attention of The Diamond Authority. According to Sapphire, it would've killed and eaten Leggy if Ruby hadn't joined them, and it ate her instead. It poofed after Ruby burned it from the inside out.