Son Goku, The Four-Tailed Monkey is one of the Bijuu spirits. It was previously hosted by the sage Roshi.
The Four-Tailed Monkey (四尾の猿, Yonbi no Saru) name was revealed as Son Goku during the time it had swallowed Naruto while under Tobi's control. It initially acted hostile towards Naruto, thinking he was going to steal his power, but calmed down when he learned otherwise, and after Naruto said he wanted to help Son.
Son revealed his title as the King of the Suiren Caves, and the Sage King of Apes, a title given to him by the Sage of Six Paths himself.