Kokuō

, 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, Kokuō 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
Kokuō 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 constructs of living chakra, which would become known as the tailed beasts. Some time after being created, the Sage sat down with all the young tailed beasts and told them that they would always be together even when separated, and that one day they would become one entity again with different names as well as forms than they did then when the time came for them to understand what true power was. Kokuō eventually fell into the possession of Iwagakure and was sealed into Han.

Appearance
Kokuō 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. During the last remaining days of the Sage of the Six Paths, Kokuō was much smaller than it is now and its horns were also shorter.

Abilities
As a tailed beast, Kokuō has a great amount of chakra and is able to use it to create a Tailed Beast Ball. Kokuō uses its horns in combat, it has shown enough ramming-force to even injure Gyūki and knock it a considerable distance away.

Shinobi World War Arc
After Han's full transformation into Kokuō, he rammed into Gyūki and pushed it a considerable ways back. Seemingly rushing towards the shinobi to attack them again, Kokuō is restrained by Tobi. As the chain wraps around its neck, Kokuō 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 Kokuō.

Preparing to attack the shinobi, Kokuō 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, Kokuō attempts to attack Gyūki again while it is preoccupied with Isobu. The beast is however sent flying by Gyūki. With this, the opposing beasts prepare a collaborative Tailed Beast Ball which is met by one from Naruto and Kurama.

As Naruto enters a deeper plain of the tailed beasts' consciousness, Kokuō introduces itself to Naruto. Later as the beast is freed from the chakra receiver it affirms Kurama's belief that Naruto was the person the Sage of the Six Paths had spoken about so long ago before being resealed into the Demonic Statue of the Outer Path.

Trivia

 * literally means 'respectful king'. This name probably comes from the name of the tenth-century-ʙᴄ Chinese . He's famous for the legend surrounding his visit to the sacred, where he visits the goddess so he can eat from her peaches of immortality. There is also an account of the King being shown a human-shaped automaton — a completely artificial, fully functional robot with internal organs and all. This might tie into Han's appearance and abilities. In Japanese, the name '穆王' is pronounced as 'bokuō', so the unusual 'kokuō' pronunciation used for the beast's name was probably meant to bring the word  to mind.
 * The kanji '穆' in Kokuō's name comes from the archaic adjective, which means 'peaceful and lovely' or 'humble and dignified'.
 * According to Kishimoto's words from the second Naruto artbook, he tried to do a mix of a dolphin and a horse.