Son Gokū

The is a tailed beast sealed within Rōshi of Iwagakure. It has been captured by Kisame Hoshigaki, then extracted and sealed by Akatsuki.

Background
According to Madara Uchiha, the Four-Tails first came into being in the waning days of the Sage of the Six Paths' life; to prevent the Ten-Tailed Beast's resurgence, the Sage split up its chakra and used Izanagi to create nine separate, monstrous constructs of living chakra, which would become known as the tailed beasts. The Four-Tails eventually fell into the possession of Iwagakure and was sealed into Rōshi at a young age.

Appearance
The Four-Tails resembles a giant monkey with red fur complete with horns on its forehead and elongated canine teeth. It also has spike-like protrusions on its four tails.

Abilities
Not much is known about the Four-Tails' power, except its ability to mix its earth and fire natures, and shoot lava from its mouth; a nature which hasn't been seen outside the realm of kekkei genkai.

Activity inside Rōshi
As the Four-Tails' jinchūriki, Rōshi could use its ability to create a torrent of lava that can melt almost anything.

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 the Four-Tails' power over lava. The Four-Tails was later extracted and sealed, leading to Rōshi's death.

Conception
The design of the Four-Tails is a combination of a gorilla and the Ōzaru from Dragon Ball, and the master of Goku, the main character of said series, was also named Roshi. 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 was based on.