Gyūki

The is an ushi-oni demon sealed within Kira Hachi of Kumogakure.

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

As it's Jinchūriki, Kira Hachi, battled Team Hawk to the point of annoyance, the Eight-Tails was released by Kira to deal them itself. As it was freed, it ordered Team Hawk to cower in fear of it; something they complied to do.

Abilities
Little is known about the Eight-Tails' strength so far.

Kira 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.

Influence
The, is a creature which appears in the folklore of Japan. The ushi-oni is a massive, brutal sea-monster with a head and body of a bull with eight tentacles of an octopus, which lives off the coast of Western Japan and attacks fishermen.

Some ushi-oni appears as protective symbols in Japanese festivals. Something like the dragon dancers at a Chinese New Year celebration, the ushi-oni are represented with a huge, multiple-person costume with a cloth body and a carved, painted head held upon a pole. They each have a sword for a tail, and is thought to drive away evil spirits.

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 dragon, Yamata no Orochi. Some fans believed that Orochimaru was the Eight-Tails 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. But, in chapter 413, it has been confirmed that that the Eight-Tails is a giant bull with eight tentacles for it's tails, which is called an Ushi-Oni.

NOTE: The Eight-Tails has only one complete horn, the other is halfway missing. Some fans believe this may be a battle scar left by Kira's mysterious brother, who was mentioned by Kira when Sasuke dodged an attack while Kira was under the Eight-Tails' chakra cloak.