Gatekeeper

The Gatekeeper was a giant fiddler crab that served as the guardian of the Moon's entrance.

Personality
The crab was both very dutiful and hostile in nature, attacking anyone or anything that came near the Moon entrance, except Toneri's puppets and presumably the Ōtsutsuki clan members.

Appearance
Though resembling a normal crab, it was enormous in size. It had a red-coloured exoskeleton with yellow and blue stripes in its eight legs and two asymmetric pincers, along with many sharp teeth. It also had a white crown-like structure in its head.

Abilities
This crab was able to use Water Release techniques such as the Water Release: Wild Water Wave technique. It also mastered a degree of genjutsu, as it was capable to trap its victims. Its enormous pincers made it capable to shred humans into pieces. It was also very durable, as its armour made the crab very difficult to kill.

The Last: Naruto the Movie
The crab suddenly appeared from beneath the cavern sediment and ambushed Shikamaru and Sai during their discussion. Later, via hard efforts, they managed to defeat it with assistance from Sakura who struck its upper carapace, crushing it along with its inner organs, expelling large amount of a yellow liquid-like substance.

Trivia

 * Sai later managed to use its form as one of his art designs when he battles a puppet armament which were guarding the artificial sun.