The tailed beasts are nine giant demons that are constructed from raw chakra and used to terrorize the mortal realm while the shinobi still banded together in clans. They are differentiated by the number of tails they have, with each possessing an amount from one to nine. The tailed beasts are large, living forms of chakra, sometimes referred to as "Chakra Monsters", giving them power that far outmatches most shinobi. However, this immense strength is tempered due to their bestial nature, which restricts them from being intelligent enough to use it effectively. When the tailed beasts are sealed within humans, they are able to benefit from the knowledge of their hosts, giving them the ability to speak and better utilize their powers.
The nine tailed beasts, as they are known, follow…
One-tailed Beast: Shrike, container is Kinjo Ai, a wandering nin.
Two-tailed Beast: Unknown, container unknown
Three-tailed Beast: Tiger, container is Kouhei, a missing-nin.
Four-tailed Beast: Badger, container is unknown but dwells within Kirigakure.
Five-tailed Beast: Plant-wolf, container is Matsu Moroha who is a genin of Sunagakure
Six-tailed Beast: Bear, container is Kurosawa Haruka who is a jounin of Kumogakure
Seven-tailed Beast: Unknown
Eight-tailed Beast: Aberration, container is Kurotou who is a genin of Konohagakure
Nine-tailed Beast: Heavily classified, container is the Sandaime Hokage, the leader of Konohagakure