Son Gokū

The is a demon formerly sealed within Rōshi of Iwagakure.

Background
It is currently unknown of what type of animal is the Four-Tails. Its Jinchūriki, Rōshi, was defeated by Kisame Hoshigaki just before his introduction. According to Kisame, he had a difficult battle because he had to face the Four-Tails' Lava Release.

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

Activity inside Rōshi
Little is known about the Four-Tails' abilities. As the Four-Tails' host, Rōshi has made use of its Lava Release techniques, which is rarely seen outside of kekkei genkai.