The Three-Tailed Giant Turtle (三尾の巨大亀, Sanbi no Kyodaigame) is a large, turtle-like tailed beast. It was formerly sealed in Yagura. The Three-Tails receives much more attention in the anime than the manga.
At some point in the past, the Three-Tails was sealed within Yagura from Kirigakure. It was later extracted from its host for some reason and escaped into the wild. Because it did not have a host it was not smart enough to effectively utilize its great strength, instead being reduced to animal instincts.