Kokuō

Kokuou, better known as the is a tailed beast sealed within Han from Iwagakure. It has been captured, extracted, and sealed by Akatsuki. In the anime, the Five-Tails is indicated to have been one of the two tailed beasts that Akatsuki captured before the start of Part II. It was later resealed into a resurrected Han during the Fourth Shinobi World War.

Background
According to Tobi, the Five-Tails first came into being in the waning days of the Sage of the Six Paths' life; to prevent the Ten-Tails' resurgence, the Sage split up its chakra and used his ability to create nine separate, monstrous constructs of living chakra, which would become known as the tailed beasts. The Five-Tails eventually fell into the possession of Iwagakure and was sealed into Han.

Appearance
The Five-Tails primarily resembles a white horse, but with a dolphin's head. It has two pointed long horns and three shorter horns in front. The ends of its horns, hooves, and tails are light brown, with some of the same-coloured spots before the brown areas of its horns and hooves. It also has red markings under its dark blue-green eyes.

Abilities
Not much is known about the Five-Tails, but since it is a tailed beast, it has a great amount of chakra and is able to use it to create the Tailed Beast Ball. The Five-Tails can also use its horns in combat, showing enough ramming-force to even injure the Eight-Tails.

Shinobi World War Arc
After Han's full transformation into the Five-Tails, he rammed into the Eight-Tails and pushed it a considerable ways back. Seemingly rushing towards the shinobi to attack them again, the Five-Tails is restrained by Tobi. As the chain wraps around its neck the Five-Tails comments on the pain and laments about being controlled in this manner, revealing that for an instant it had broken free of Tobi's control and attempted to attack him. Forced back into its submissive Version 2-like state, it begins to attack the shinobi as commanded. After Son Gokū is resealed into the Demonic Statue of the Outer Path, Tobi, who is intent on going all-out from this point on, forces Han to fully transform into the Five-Tails.

Preparing to attack the shinobi, the Five-Tails along with the other four beasts create Tailed Beast Balls. These attacks however, are deflected by Naruto and they are rebuffed by a now fully-transformed Naruto. Wasting no time in their attacks, the Five-Tails attempts to attack the Eight-Tails again while it is preoccupied with the Three-Tails. The beast is however sent flying by the Eight-Tails. With this, the opposing beasts prepare a collaborative Tailed Beast Ball which is met by one from Naruto and the Nine-Tails.

Trivia

 * According to Kishimoto's words from the second Naruto artbook, he tried to do a mix of a dolphin and a horse.