Kokuō

, more commonly known as the, is one of the nine tailed beasts. It was last sealed within Han from Iwagakure.

Background
Kokuō first came into being in the waning days of the Sage of the Six Paths, who used his Creation of All Things ability to separate the Ten-Tails' chakra from its body and divide it into nine separate constructs that would come to be known as tailed beasts in order to ensure that it would never resurface after his death. 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 and forms than they did. Then, when the time came, they would know what true power is. Kokuō eventually fell into the possession of Iwagakure and was sealed into Han, but was captured and extracted by Akatsuki.

Personality
Kokuō seems to be a quiet and reserved individual. Though it does not speak much, Kokuō uses the rather old-fashioned when referring to itself, causing it to come across as very polite. It also has pride as a tailed beast, being dismayed at being controlled and being used as Tobi's puppet during the Fourth Shinobi World War. After the end of the war, Kokuō declared that it wants to seclude itself in the forest, which implies it is a pacifistic individual, not wanting confrontations anymore.

Appearance
Kokuō primarily resembles a white horse, but with a dolphin's head. It has two pointed long horns and three (two in the anime) 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 for attacks.

Its willpower also seemed to be incredibly strong, as it was the only tailed beast who was able to break free, albeit temporarily, of Tobi's control and attempted to attack the masked man.

Physical Prowess
Kokuō possesses great physical strength and while using its horns in combat, it has shown enough ramming-force to injure Gyūki, and knock the beast a considerable distance away.

Nature Transformation
By combining Water Release and Fire Release to create Boil Release, Kokuō is also able to increase the temperature of chakra to its boiling point. This ability forms the basis of what is referred to as "steam-based ninjutsu" granting itself, or its jinchūriki overwhelming physical strength. When in use, this ability causes the jinchūriki to emit copious amounts of steam from their body.

Shinobi World War Arc
After Han's full transformation into the Five-Tails, he rammed into Gyūki 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, 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, the Five-Tails attempts to attack B again while it is preoccupied with the Three-Tails. The beast is however sent flying by B, who anticipated the attack. 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 plane 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.

Ten-Tails Revival Arc
Kokuō appeared alongside the other tailed beasts with the exception of Gyūki and Shukaku when the Ten-Tails looked down at Naruto. The image of the tailed beasts, along with a silhouette of the Sage of the Six Paths, drove the Ten-Tails to a fit as it evolved further. Through the combined effort of the Allied Shinobi Forces, it was later possible to pull Kokuō out of Obito, freeing it alongside with the other tailed beasts.

As Naruto in Tailed Beast Mode pinned Madara down with a tail, Kokuō and the other tailed beasts joined in and did the same to increase the damage, but the Uchiha survived their attacks and retreated to a safe distance. After he received his own right Rinnegan eye from a White Zetsu clone, Madara quickly summoned the Demonic Statue of the Outer Path from Obito's body, and used the Rinnegan to repel the tailed beasts. With the tailed beasts stunned, Madara then proceeded to chain them up and prepared to seal them back into the statue. Ultimately, Kokuō, along with the other tailed beasts, are all resealed within the Demonic Statue.

Final Arc
Later, the chakra Kokūo transferred to Naruto earlier manifested within Naruto's subconscious along with the other tailed beasts to speak with the young ninja and the spirit of Hagoromo Ōtsutsuki. There, they voiced their shared belief that Naruto is the child of prophecy.

After Madara's ambitions were cut short by Black Zetsu, who revealed to be the silent manipulator of ninja history since it was first created by Kaguya Ōtsutsuki, the black entity used Madara's body as a medium to resurrect its creator. During the battle in Kaguya's snowy-filled dimension, Kokuō used its chakra to greatly boost Naruto's offensive power, ploughing through Kaguya's defences and momentarily overwhelming her. Ultimately, Kaguya Ōtsutsuki who had taken control of Madara was defeated by Naruto Uzumaki and Sasuke Uchiha and sealed using the Six Paths Chibaku Tensei technique. The beasts were all ejected from the statue and then summoned back to the real world by Hagoromo and the previous Kage. After the past Kage return to the after life, the tailed beasts begin discussing their plans for the future now that they are finally free, Kokūo expressing its desire to seclude itself within the forests. Sasuke however makes his plans known. Desiring to kill the current Kage and seize control of the tailed beasts for his plans of revolution in the shinobi world, he quickly accomplishes the latter with his Rinnegan. With the tailed beasts subdued by his genjutsu, Sasuke then seals them away within his Chibaku Tensei.

Video Games
Although Kokuō itself is not playable, Han takes on its form in both his Ultimate attack and as his awakening form in Naruto Shippūden: Ultimate Ninja Storm 3 and Naruto Shippūden: Ultimate Ninja Storm Revolution.

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' and are also traits often associated with horses and dolphins.
 * According to Kishimoto in the second Naruto artbook, he tried to do a mix of a dolphin and a horse when he created Kokuō.