Gyūki

The is currently one of the two remaining Tailed Beasts that were not extracted by Akatsuki, the other one being the Nine-Tailed Demon Fox.

Abilities
Most of his abilities are still yet unknown, but it has been shown that Kira Hachi uses seven swords in a very peculiar manner. He places one on his neck (right cheek), one in his mouth, one in his left armpit, two in his elbows, one on his right hip and the last one in his right knee. This special maneuver allows him to attack his opponent with great speed and also in a circular faction, like a ball. Also, he is able to put chakra into his blades allowing them to up their power and also retransfer any other chakra being propelled by the opponent. So far, he has supernormal strength and an unorthodox style of being able to wield seven swords at the one time. In addition, he is able to use the Eight-Tails' chakra to break illusions, even ones casted from a Mangekyo Sharingan. He also has control over his inner demon.

Speculation
As most of the other Tailed Beasts seen so far are based upon mythological creatures, it was speculated that this Tailed Beast was based upon the eight-headed and eight-tailed serpent, Yamata no Orochi. Some fans believed that Orochimaru was the Eight-Tailed Beast due to his snake-like abilities, however, this was proven false in chapter 408. However judging by the characteristics he possesses such as his dislike of light it has been speculated that the Eight-Tails is perhaps a mole, others think it might be a bee. But, based on his silouettte, he may also be an octopus of some kind. In chapter 413, it has been revealed that that the Eight-Tails is a giant bull with eight tentacle-like tails, which is called an Ushi-Oni.