Kokuō

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

Background
Kokuō and the tailed beasts first came into being in the waning days of Hagoromo Ōtsutsuki, who used his Creation of All Things Technique to divide the Ten-Tails' chakra into nine separate bodies and giving each a name. Some time after being created, the Sage explained to the young tailed beasts that a link existed between them despite their separation. He also foretold of a day when they would be brought together again - though not as they were - and that at that time someone would emerge to show them what true power is. After Hagoromo's death, the tailed beasts drifted apart, apparently due to Kurama's belief that their number of tails was an indication of their strength. Kokuō, after living in an unspecified forest, eventually fell into the possession of Iwagakure during Mū's era, 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 tan, with some of the same-coloured spots before the tan 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 Six Paths, Kokuō was much smaller than it is now and its horns were also shorter.

Abilities
As a tailed beast, Kokuō possesses a massive amount of chakra and can use it to create a Tailed Beast Ball. Its willpower was incredibly strong, as it was the only tailed beast who broke free, albeit briefly, of Tobi's control and attempted to attack the masked man. Kokuō also possesses great physical strength and uses its horns in combat, showing enough ramming-force to injure and knock Gyūki a considerable distance away.

Nature Transformation
By combining water and fire-natured chakra 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.

Fourth Shinobi World War: Climax
After Han's full transformation into the Five-Tails, he rammed into the Eight-Tails and pushed it a considerable ways back. Seemingly rushing toward the shinobi to attack them again, the Five-Tails was restrained by Tobi. As the chain wrapped around its neck, Naruto and Killer B heard the Five-Tails' voice lamenting about the pain of being subdued by Tobi, revealing that for an instant it had broken free of the masked man's control and attempted to attack him. Forced back into its submissive Version 2-like state, it began to attack the shinobi as commanded. After Son Gokū was resealed into the Demonic Statue of the Outer Path, Tobi, who was intent on going all-out from that point on, forced Han to fully transform into the Five-Tails.

The opposing tailed beasts prepared to attack the shinobi, but they were rebuffed by a fully-transformed Naruto. Wasting no time in their attacks, the Five-Tails attempted to attack the Eight-Tails again while it was preoccupied with the Three-Tails, but was sent flying by the ox, who anticipated the attack. The opposing beasts regrouped and then prepared a collaborative Tailed Beast Ball, which was met by one from Naruto and Kurama.

As Naruto entered a deeper plane of the tailed beasts' consciousness, Kokuō introduced itself to the young shinobi. Later, as Kokuō was freed from the chakra receiver, it affirmed 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.

Birth of the Ten-Tails' Jinchūriki
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 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.

When Madara ran towards the tailed beasts, both Shukaku and Gaara conjured numerous sand bullets and were able to restrain the Uchiha's movements. Using the opportunity to attack, the tailed beasts each took a swing at their opponent, giving sand-manipulating duo an opening to use their Desert Layered Imperial Funeral Seal, but Madara managed to escape the sand pyramid with his Susanoo. 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 chained them up and sealed them back into the Demonic Statue.

Later, the chakra Kokuō transferred to Naruto earlier, manifested within Naruto's subconscious along with the other tailed beasts to speak with the young jinchūriki and the spirit of Hagoromo Ōtsutsuki. There, they all voiced their shared belief to the Sage that Naruto was the child of prophecy.

Kaguya Ōtsutsuki Strikes
After Madara's ambitions were cut short by Black Zetsu, who revealed to be the silent manipulator of shinobi 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 against the tyrannical princess, Kokuō and the other tailed beasts continued to support Naruto by giving him their combined power to overwhelm her. Ultimately, Kaguya was defeated by Naruto and Sasuke, who used the Six Paths — Chibaku Tensei to eject all the tailed beasts from her and seal her away inside a newly-created moon within her own dimension.

After the spirits of Hagoromo and the past Kage summoned both the tailed beasts and Team 7 back to the real world, the tailed beasts began to discuss their plans for the future since they were finally free, with Kokuō expressing its desire to seclude itself within the forests. Sasuke, however, made his own intentions known: removing as many people as possible who could get in the way of his plans of leading the world through a revolution to do away with current shinobi systems, including killing all the current Kage and tailed beasts. Just as Sasuke swiftly subdued the tailed beasts with his Rinnegan and sealed them within his Chibaku Tensei, Naruto reassured the tailed beasts that he would save them once again. After Naruto was able to defeat and reform Sasuke, the two dispelled the Infinite Tsukuyomi and Sasuke released Kokuō, along with the other tailed beasts, from their prisons.

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.