Tailed beasts

The tailed beasts are nine giant demons that serve to drive the plot of the Naruto series. They are differentiated by the number of tails they have, with each possessing an amount from one to nine. The beasts are large, living forms of chakra, giving them power that far outmatches most ninja. However, this immense strength is tempered due to their bestial nature, which restricts them from being smart enough to use it effectively. Beasts sealed within another person are able to benefit from the knowledge of their hosts, giving them the ability to speak and better utilize their powers. Akatsuki, a criminal organization that becomes one of the primary antagonists of the series, spends much of their screentime trying to capture the first nine beasts or their hosts. Once captured, Akatsuki's members seal the beast away for future use. The Akatsuki leader Pain, claims these beasts will facilitate in the creation of a weapon capable of destroying an entire country whilst the group's true leader Madara reveals his intention of merging the tailed beasts into their original form, the ten-tailed beast.
Only some of the beasts are actually seen in the series, and those that appear are not fully explained. The second Naruto artbook, released in July of 2009, provides an explanation of the beasts and their hosts. The beasts are, in numerical order: Shukaku, the one-tailed tanuki (sealed within Gaara of Sunagakure); the two-tailed monster cat (sealed within Yugito Ni'i of Kumogakure); the three-tailed turtle (previously sealed within Yagura of Kirigakure); the four-tailed gorilla, based on the Great Ape from Dragon Ball (sealed within Rōshi of Iwagakure); the five-tailed dolphin-horse (sealed within Han of Iwagakure); the six-tailed slug (sealed within Utakata of Kirigakure); the seven-tailed beetle (sealed within Fū of Takigakure); the eight-tailed ushi-oni-like beast (sealed within Killer Bee of Kumogakure); and the nine-tailed demon fox (sealed within Naruto Uzumaki of Konohagakure). As of the latest Naruto chapter, only the eight-tailed and the nine-tailed beasts have yet to be captured by Akatsuki.