Tailed Beast

In the Naruto universe there are at least nine demons called the tailed beasts (尾獣 bijū) inhabit the planet. The organization Akatsuki, has a sinister plan that involves the capture of the tailed beasts.

History
It seems that the tailed beasts were around before the First Great Ninja War. The shinobi nations first involved these demons by attempting to capture them and harnessing their powers to defeat their enemies. The attempt was impossible as no nation was able to control such power. Instead, the nations moved to capture the beasts and seal them a human, a jinchūriki (人柱力, lit. Power of Human Sacrifice). This method worked and by the time peried of the series, the only known tailed beast to have not been sealed was the Three Tails.

Jinchūriki
The jinchūriki or host, exhibit extraordinary powers, and in some cases are stronger than their tailed beasts for the very fact that they are able to control their power. Constrast to this if a tailed beast goes to long without a host, it can lose its intelligence and become nothing more than a giant animal thus making it weaker. The hosts usually show some physical characteristics of the tailed beast within them and the beasts traits can become infused with the hosts personality. According to Akatsuki, the hosts tend to be lonely people who loathe humanity, which was proven true when the first two beasts sealed by the organizaiton was done with almost little effort from their ninja villages, infact they were glad to be rid of their jinchūriki. The tailed beasts also are protective of their hosts for the soul reason that if the host dies so does the demon inside. Likewise, if the tailed beast is removed from the host, the host would die. So far there is only two known was to seal a tailed beast into a host. One Dead Demon Consuming Seal, which the Fourth used to seal the demon fox into Naruto, and Akatsuki's Illusionary Dragon Nine Consuming Seals, a three day long jutsu that with the combined effort of all Akatuski members forces the tailed beast out of its host and into a nine eyed statue. Akatuski has so far captured seven tailed beasts.

One-Tailed Shukaku

 * Name: One-Tailed Shukaku (一尾の守鶴 Ichibi No Shukaku) or Shukaku of the Sand (砂の守鶴 ''Suna no Shukaku)
 * Creature Type Tanuki (racoon-dog, native only to Japan and other Asian countries)
 * Host None, Formerly Gaara and previously two unknown Sunagakure citizens.
 * Unique traits: Ability to spit concentrated balls of wind mixed with chakra
 * Statues: Caputed, extracted, and sealed by Akatsuki

Shukaku is said to be the corrupted spirit of a Sand monk turned demon. He was originally sealed inside a teakettle and then placed inside his first host, similar to the tale of Bunbuku Chagama. He was sealed inside Gaara by Chiyo on the orders of Gaara's father, the Fourth Kazekage, using Gaara's mother as the sacrifice. Shukaku had two other hosts before Gaara, both of whom died when Shukaku was extracted from them. Shukaku is intelligent but also incoherent, giving him the demeanor of a drunkard, this fits well with the typical depictions of tanuki in Japanese folklore. Shukaku itself can translate to "drunk" in Japanese. Like other tailed beasts, its innate rage and bloodlust influence its behavior, and it relishes in the opportunity to kill anyone in sight. As a host of Shukaku, Gaara has the ability to move and levitate sand at will, and is protected at all times by the Shield of Sand, regardless of his will (even self-inflicted injuries are prevented by the sand). He also suffers from lifelong insomnia, a side effect of being Shukaku's host. Should he fall asleep, Shukaku would eat away his personality making him a monster. When angry or particularly bloodthirsty, Gaara begins to transform into a human-sized version of the Shukaku by layering sand over his body. The sand takes on the appearance of the Shukaku, and enhances his abilities greatly. While in this form, Gaara gains immense strength, endurance, and the ability to use Sand Shuriken. The weak point of this transformation is at the base of its tail, where any strong enough blow (such as an exploding note) will cause severe damage to it. He can also stretch his sand limbs as he does with his regular sand, the primary difference being that his limbs remain just as strong as when they are retracted. This transformation seems to occur very slowly, as both attempts to assume this form during the Chunin exams were foiled before completion. Gaara has only been able to reach a point at which all but his legs have been covered in sand. In dire situations, Gaara can create a life-sized replica of Shukaku from sand. Unlike his human-sized version, this replica can be created almost instantly. Gaara controls the replica from within, remaining safe from enemy attacks. Despite being made of sand, Gaara seems to be unable to regenerate any damage caused to the replica. While in this form, Gaara can also release Shukaku's spirit, allowing the demon to use his full power. To do this, Gaara must emerge from within the replica, settling on the top of its head, and force sleep upon himself. Once released, Shukaku can inhale deeply, distending its own stomach, and punch his stomach to use Wind Release: Drilling Air Bullet (風遁・練空弾 Fūton: Renkūdan?), shooting a large ball of compressed air and chakra at an opponent that destroys all in its path. Because Gaara protrudes from the top of Shukaku's head, he can be awoken, thus suppressing Shukaku's spirit once more. In Part II, Deidara of Akatsuki is assigned to capture Shukaku. After defeating Gaara in combat and taking him to Akatsuki's lair, Shukaku is forcefully extracted from Gaara, thus killing him. Gaara is resurrected through a combination of Chiyo's life-transferring technique and Naruto's chakra. Gaara's ability to control sand still appears to be intact, though to what degree remains unclear.

Two-Tailed Cat

 * Name: Two-Tailed Demon Cat ニ尾の猫俣 Nibi no Nekomata
 * Creature type: Nekomata (magical, two-tailed cat)
 * Host: None, formerly Yugito Nii
 * Unique traits: Fire-breathing
 * Status: Captured, extracted, and sealed by Akatsuki

The two-tailed demon cat is a fire-breathing, flaming cat sealed within [Yugito Nii]] of Kumogakure. Soon after her introduction Yugito is captured by Akatsuki members Hidan and Kakuzu, though it is unknown which was specifically assigned to capture her. Despite luring her two pursuers into a closed space, sealing the exits, and transforming herself into the demon cat, Yugito was overpowered and rendered unconscious by the two Akatsuki members. Zetsu later arrives to take her off their hands, and she later dies as a result of having the beast extracted from her.

Three-Tails

 * Name: Three Tails 三尾 Sanbi
 * Creature TypeUnknown (turtle-like creature)
 * Host: None
 * Unique traits:Unknown
 * Status: Captured and sealed by Akatsuki.

The three-tailed beast is a gigantic, turtle-like demon that Akatsuki assigns Tobi (with the help of Deidara) to capture. This creature may be based on the Genbu or the Kappa. What abilities this creature possesses remains unclear, as it did little to actually attack the two in its short appearance. It is clearly quite fast despite its size, as evidenced by its ability to keep up with Tobi, who was running on the water at the time. With the three-tails distracted, Deidara molded an explosive clay fish in an attempt to capture it. It's unclear if this marked the end of the battle, as the remainder of the battle goes unseen. As the two towed the beast to the Akatsuki lair after the battle, Tobi gloated of bringing the three-tails down with his special jutsu, but Deidara thought otherwise. It is later sealed in the sealing statue. Unlike the other tailed beasts seen thus far, the three-tails lacked a host. It is different from the Shukaku and the demon fox as it isn't intelligent, which makes it little more than a wild animal. According to Deidara, lacking a host made the beast weaker because it wasn't intelligent enough to control its own strength.

Four-Tailes

 * Name: Four Tails (四尾 Yonbi)
 * Creature type:Unknown
 * Host: None, formerl an unnamed elderly man
 * Unique traits:Access to many types of elemental chakra

The four-tails jinchūriki is an elderly man of unknown origin who was eventually captured by Kisame Hoshigaki. Reportedly, it was a difficult battle for Kisame, who had to face many elemental fusions, rarely seen outside of kekkei genkai. The four-tailed beast is sealed before the end of the next chapter, leading to the old man's death.
 * Note: Able to use multiple elemental fusions.

Nine-Tailed Fox Demon

 * Name: Nine-tailed Demon Fox (九尾の妖狐 Kyūbi no Yōkō)
 * Creature type: Fox
 * Host: Naruto Uzumaki
 * Unique Traits: Near limitless chakra, massive strength
 * Status:Active, sealed in host.

The nine-tailed demon fox is a truly powerful beast; a single swipe of one of its nine tails can raise tsunamis and flatten mountains. After the demon fox attacked Konohagakure, the Fourth Hokage sealed it into the newborn Naruto by using the Dead Demon Consuming Seal, sacrificing his own life to do so.

The demon fox is a beast of almost pure malevolence. It is intelligent, and has a scathing and sadistic personality. However, it has a sense of honor, and seems to possess a respect (if merely a loathing one) for Naruto and the Fourth Hokage. Additionally, it has a distinct sense of pride and became rather angry and scornful at Naruto when he rejected its chakra in Part II, though this might have merely been anger over missing an opportunity to partially usurp control over Naruto's body.

As the demon fox's host, Naruto has accelerated healing, almost unrivaled stamina, and access to the demon fox's immense amount of chakra. The near-limitless chakra allows Naruto to perform his signature jutsu, Shadow Clone Technique, seemingly without limit. He is even able to summon Gamabunta the "Toad Boss", a normally Kage-level feat requiring an amount of chakra proportionate to the size of the summoned creature.

Naruto typically accesses this prodigious supply of chakra from the demon fox during extreme emotional states or life-threatening situations. When he first accessed it, the demon fox's innate rage took over Naruto's mind, causing him to strike out at whatever was near him at the time. Afterwards, Naruto learned to exercise more control over the chakra, and he can now retain his mental state when drawing upon small amounts of the demon fox's chakra. After Jiraiya's training, Naruto learns to summon a small degree of the demon fox's power at will. To do this, Naruto travels into his own mind (depicted as a sewer) and demands that the demon fox give him its chakra. He calls it "rent". The demon fox seems to comply simply because it is amused by Naruto's bravery, as well its own vested interest in Naruto's continued survival.

The demon fox's chakra is not without its drawbacks, however. As Naruto draws on more of the demon fox's chakra, his own personality becomes supplanted by the fox's. The degree to which this occurs is proportionate to how much chakra Naruto draws upon. The effect on Naruto's personality becomes significant once he begins accessing his "tailed" transformation states. The tailed transformations appear as fox-shaped auras of red chakra surrounding him. The number of "chakra tails" that extend from the aura indicate its power level, and its effect on his personality. In the one-tailed to three-tailed states, Naruto's personality remains dominant, but he acquires some animalistic behaviors, such as standing on all fours, fighting mercilessly, and a distinct bloodlust. In his four-tailed state (the highest seen so far), the demon fox's personality becomes dominant. Naruto becomes immensely powerful and destructive in this state, enough so to overwhelm even Jiraiya and Orochimaru. Unfortunately, this state also leaves him unable to distinguish between friend and foe which was shown when he attacked Sakura. Such a large amount of chakra requires stability, so it densely molds around him. As Yamato stated, his body is just a "skeleton" for the chakra. The intense aura created from it both shields and harms him, burning off all his exposed skin and causing him to bleed from every pore. This, combined with the accelerated cell regeneration, weakens Naruto and shortens his lifespan. In the event of the demon fox manifesting, Naruto's sensei, Jiraiya, prescribes a special written seal that is to be placed on Naruto's body. This seal suppresses the demon fox's chakra and restores Naruto to his normal self. In addition, Yamato has demonstrated a technique that will reverse Naruto's demon fox transformation, enhanced greatly by the First Hokage's necklace that Naruto wears.

The Third Hokage and Kakashi Hatake allude that if Naruto loses all control of the demon fox's chakra, the seal will break and the demon fox will be freed. Though the seal is unlikely to break on its own, the chances of it happening increase as Naruto draws upon the fox's chakra. Soon after the start of Part II, Jiraiya mentions that the seal may be becoming weaker over time. This is evident in Naruto's transformations, which occur much quicker and with greater intensity than they did in the first part of the series, often against his own will. As has been seen during Naruto's training sessions, even the slightest amount of frustration on Naruto's part allows the demon fox's chakra to leak out in excess, and Naruto rapidly progresses to a four-tailed state.

In Part II, it is revealed that the demon fox is familiar with the Uchiha clan. The precise relationship between the clan and the fox is unclear as of now, but the fox mentions a Madara Uchiha, who possessed chakra more evil ("cursed", as described by the fox) than the fox's own, a trait which Sasuke seems to share. Sasuke also demonstrates the ability to forcefully suppress the demon fox's chakra. To parallel the fox's apparent connection with the Uchihas, Itachi Uchiha of Akatsuki is originally assigned to capture it as part of the organization's goal of obtaining all the tailed beasts, but this task is later given to Pein.