Tailed Beast

In the Naruto universe, there are nine demons called the that inhabit the planet. The organization Akatsuki, has a 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 immense chakra to defeat their enemies. The attempt was a failure as no nation was able to control such power. Instead, the nations moved to capture the beasts and seal them in a human, a. This method worked and by the time period 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. In addition, if a tailed beast goes too 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 beast's 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. In fact, they were glad to be rid of their Jinchūriki. The tailed beasts are also very protective of their hosts for the sole reason: 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 methods to seal a tailed beast into a host. One, the Dead Demon Consuming Seal, which the Fourth Hokage used to seal the Nine-Tailed Demon Fox into Naruto, and the technique that Chiyo used to seal the Shukaku into Gaara. The only known technique to extract a tailed beast is the Akatsuki's Illusionary Dragon Nine Consuming Seals, a three day long jutsu that with the combined effort of all the Akatuski members, forces the tailed beast out of its host and into a nine-eyed statue. The Akatuski has so far captured seven tailed beasts.

One-Tailed Shukaku
The One-Tailed Shukaku was one of the tailed beasts that inhabited the Land of Wind prior to the start of the series. Its only known host was Gaara, but was later captured and sealed by the Akatsuki.
 * Name:
 * 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
 * Status: Captured, extracted, and sealed by the Akatsuki

Two-Tailed Demon Cat
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 two 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.
 * Name:
 * Creature type: Nekomata (Magical two-tailed cat)
 * Host: None, formerly Yugito Nii
 * Unique traits: Fire-breathing
 * Status: Captured, extracted, and sealed by the Akatsuki

Three-Tails
The Three-Tailed beast is a gigantic, turtle-like demon that Akatsuki assigns Tobi (with the help of Deidara) to capture. 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.
 * Name:
 * Creature Type: Turtle-like creature
 * Host: None
 * Unique traits: Unknown
 * Status: Captured, and sealed by the Akatsuki.

Unlike the other tailed beasts seen thus far, the Three-Tails lacked a host. According to Deidara, lacking a host made the beast weaker because it wasn't intelligent enough to control its own strength.

Four-Tails
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.
 * Name:
 * Creature type: Unknown
 * Host: None, formerly an unnamed elderly man
 * Unique traits: Access to many types of elemental chakra
 * Status: Captured, extracted, and sealed by the Akatsuki

Nine-Tailed Demon Fox
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. The demon fox appeared as an age-old natural disaster in an area where human malice festered. After the demon fox attacked Konohagakure, the Fourth Hokage sealed it into his newborn son, Naruto, by using the Dead Demon Consuming Seal, sacrificing his own life to do so. It is said however that Madara Uchiha had the power to control the demon fox, and it was believed Madara, or any Uchiha as far as Konoha's council was concerned, was the one behind the attack on Konohagakure When consumed with strong emotion or physical damage to Naruto's body, the demon fox's traits appear within him, and he eventually completely loses control of himself.
 * Name:
 * Creature type: Kitsune (Magical many tailed fox)
 * Host: Naruto Uzumaki
 * Unique Traits: Near-limitless chakra, massive strength
 * Status: Active, sealed in host.

Legend of the Tailed Beasts
According to some fans, the "Legend of the Tailed Beasts" is an epic story from Japanese folklore which Naruto author Masashi Kishimoto draws from to create the tailed beasts in his work. However it is an example of internet-based fakelore which originated on Chinese websites, some suspect this chinese site.

The veracity of the legend breaks when it is cross-referenced with non-fandom sources on Japanese folklore. For example, the isonade, which is held to be the three-tailed beast in this account, is in fact equipped with only one large, hook-covered tail, and the "houkou" which is actually a Chinese creature called penghou, named as the five-tailed beast, has no tail at all. The badger was said to be the lord of all these beasts in the original legend, and Naruto fans have claimed certain Bijuu to be a badger instead of it's true form which was a tanuki for Ichibi no Shukaku and a bat for Shichibi no Makumore)

Additionally the author of the story admitted it was not a true legend, unfortunately it has gone largely unnoticed by most fans of the story.