Zero-Tails

The is a Noh masked leech-like spirit sealed within Amaru. It only appears in Naruto Shippūden 2: Bonds.

The Zero-Tails is an evil spirit created by the dark thoughts and feelings of the downtrodden people. This parasitic creature will seek out and feed upon any negative emotions within one's heart and allows the spirit to create dark chakra. When anyone near the Zero-Tails is emotionally unstable, it will inhabit their body and take them over. The process of implanting the Zero-Tails into a human host does not require a human sacrifice, nor did its extraction kill the host.

The remnants of the Land of the Sky used the Zero-Tails to discretely create enough dark chakra to power their ultimate weapon, "Ancor Vantian". This dark chakra was also personally used by Shinnō, an agent of the Land of the Sky and the man who orchestrated the revival of the Sky ninja forces.

Abilities
It has the ability to fire tendrils from its body and to also form them into hands. It seems to also be able to regenerate since its head reattached after being ripped off by Naruto Uzumaki in his four-tailed form. It also has a very large mouth that appears under its mask. Large molars grow from inside its mouth and it seemed to have tried to bite Sakura Haruno when she attacked it. It appears to have the ability to read the hearts of others, seeing as it saw Naruto's urge to rescue Sasuke Uchiha, and his pain when he was unable to bring his friend back.

Trivia

 * Despite the beast referring to itself as Zero-Tails and Shinnō referring to it as a tailed beast, it is not considered one of the original tailed beasts due to the fact that it was not born from the Ten-Tailed Beast. It's also noteworthy to comment that, during the movie, when the Zero-Tails latched onto Naruto, the Nine-Tailed Demon Fox reacted aggressively to its presence and attempted to expel it.