Son Gokū (Ten)

The Four-Tailed Giant Monkey (四尾の猿, Yonbi no Saru) is a tailed beast sealed within Ryun Mazuka of Yamagakure. It was formerly sealed within Rōshi of Iwagakure and was captured by Kisame Hoshigaki, then extracted and sealed by Akatsuki. After Akatsuki fell, the Four-Tails fell into the hands of Yamagakure, who sealed it within Ryun.

Background
According to Madara Uchiha, the Four-Tails came into being near the end of the waning days of the Sage of the Six Paths' life; to prevent it's resurgence, the Sage split up the demon's chakra of the Ten-Tailed Beast into nine separate, monstrous constructs of living chakra, which would become known as the tailed beasts. The Four-Tailed Monkey eventually fell into the possession of Yamagakure and was sealed into Ryun at a young age.

Activity Inside Ryun
Ryun primarily relies on his mastery of Lava Release, of which he has a vast arsenal due to the Four-Tails. With this arsenal, he can attack or defend against his opponents in numerous manners. In addition, he has proven to be very adept in taijutsu, which, combined with his impressive speed and his jinchuriki forms, allows him to effectively catch his opponents off guard and evade attacks.

Ryun is one of the few jinchūriki that is able to fully control their tailed beast. Like other jinchūriki, Ryun has the ability to create a chakra cloak by drawing on part of his demon's power. He can then form this chakra into the form of two monkey horns, and use them to attack the enemy; Or he can partially transform into his tailed beast and do the same. This attack has incredible speed and power, but the linearity of his movements makes him easier to predict by dojutsu such as Sharingan and maybe Rinnegan. Because he has a strong relationship with his own tailed beast, it acts as his partner in battle, disturbing his chakra to dispel even genjutsu.

Like his his cousin, Tora Mazuka, Ryun is able to become his tailed beast itself with great ease. Also like Tora, he can retain complete control even in this fully released state, and partially transform parts of his chakra shroud into parts of the Four-Tails body. When he fully transforms, he can easily and repeatedly fire massive chakra blasts many times the size of its already giant body.