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 into 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.

Activity inside Rōshi
As the Four-Tails' jinchūriki, Rōshi had the ability to use its power of Lava Release, a combination of fire and earth the likes of which have rarely been seen outside of kekkei genkai.

Hunt for Itachi Arc
After an unseen and difficult battle with Kisame Hoshigaki, Rōshi was somehow defeated and captured by him. Kisame had difficulty capturing Rōshi because of the Four-Tails' ability of Lava Release. 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.