Son Gokū

, better known as the is a tailed beast sealed within Rōshi of Iwagakure. Son Goku is the ape king of the Suiren Caves, and refers to itself as the Sage King of the Apes. It was captured by Kisame Hoshigaki, then extracted and sealed by Akatsuki. Son was later resealed into a resurrected Rōshi during the Fourth Shinobi World War.

Background
According to Tobi, the Four-Tails first came into being in the waning days of the Sage of the Six Paths' life in order 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. Son Goku eventually fell into the possession of Iwagakure and was sealed into Rōshi at a young age.

Personality
From what has been seen of Son Goku, it detests being called by the name "Four-Tails". It is also a very prideful tailed beast boasting about who it was and its lineage calling itself "holiness of all the heavens". It also seems to detest humans a great deal believing that apes are more intelligent that humans. This hatred seems to stem from that fact that humans have enslaved it. Like all apes however, it does tend to gibber.

Appearance
Son Goku primarily resembles a red-furred and green-skinned monkey, but with a muscular body of a gorilla. Its eyes are yellow and the pupils are white. It also has horns on its forehead, elongated canine teeth, and spike-like protrusions on its tails. The fur on its chin and on the top of its back resemble Rōshi's beard and hairstyle.

Abilities
Son Goku has the ability to simultaneously use the earth and fire natures, to spit lava from its mouth; something that hasn't been seen outside the realm of kekkei genkai. Since it is a tailed beast, it is able to use the Tailed Beast Ball. Son Goku is also shown to be very strong as it was able to lift and toss the Eight-Tails.

Activity inside Rōshi
As Son Goku's jinchūriki, Rōshi could borrow its ability to create and use lava.

Itachi Pursuit Arc
After an unseen and difficult battle with Kisame, Rōshi was somehow defeated and captured by him. Kisame had difficulty capturing Rōshi because of Son Goku's power over lava. Son Goku was later extracted and sealed, leading to Rōshi's death.

Shinobi World War Arc
During the Fourth Shinobi World War, Tobi forces Rōshi into his full tailed beast transformation. With Son Goku on the battlefield, the great ape immediately attacks the Eight-Tails even managing to grab it by its tentacles and flip the beast over. It later attempts to seemingly swallow Naruto, but as the young jinchūriki attempts to free himself from between the ape's jaws, Son opens his mouth even wider and swallows him.

Now inside the beast, a chained up Son asks Naruto if he had come to steal its power as well, and lashes out at him when he refers to it as the "Four-Tails" telling him that it was the king of the apes and would be addressed with due respect. As their conversation continues, Son finds Naruto to be an open, honest person which shocked him somewhat. After cursing that all humans did was seal them away and deny their existence, Naruto shares that he too was treated like that once and hated to see someone who did not care who they were controlling the beasts in this manner. After laughing at the prospect of Naruto wanting to be friends with his tailed beast, it realises that he is serious and tells the boy that there might be a way to stop him if the chains that bound it were undone.

Conception
The design of Son Goku is a combination of a gorilla and the Ōzaru from Dragon Ball. The horns on its forehead are modelled after the diadem worn by Sun Wukong from the classic Chinese novel Journey to the West, the character that Goku who could also transform into an ape was based on. And his name Son Goku is directly inspired from Sun Wukong.

Trivia

 * Likely owing to the inspiration Son takes from Journey to the West, when Naruto meets him within Rōshi, he is sealed to a giant stone. This reflects the mountain under which Sun Wukong was sealed.