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.

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

Abilities
Nothing so far is known about the Eight Tails's 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.

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.