Son Gokū

The is a demon monkey sealed within Rōshi of Iwagakure. Little is known about the Four-Tails, only that it allowed Rōshi to use its Lava Release abilities, a combination of fire and earth the likes of which have not been seen outside kekkei genkai. Kisame Hoshigaki of Akatsuki had difficulty capturing Rōshi because of this. 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. The horns on its forehead are modelled after the diadem worn by Sun Wukong from the classic Chinese novel Journey to the West.