One-Tailed Shukaku
The One-Tailed Shukaku (一尾の守鶴 Ichibi no Shukaku?, English TV "Shukaku the Sand Spirit") has been in the possession of Sunagakure for many years, being sealed into two previous citizens before being sealed in Gaara. Chiyo sealed it into Gaara under the Fourth Kazekage's orders, and Gaara's mother was used as a sacrifice for the sealing process. 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. 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. He is also given constant protection by his sand, as it surrounds him to block any and all damage regardless of his will. As a symbol of the beast sealed within him, Gaara has Shukaku's characteristic rings around his eyes. Although they are a part of his physical appearance, their presence is further amplified by his lifelong insomnia; should he fall asleep, Shukaku will supplant his personality and take control of his body. If need be, Gaara can bring out Shukaku in battle to deal with more powerful opponents; Gaara first creates a life-sized replica of Shukaku from sand that he is merged with at its forehead. He then forces himself to sleep to release the demon at its full power. Once released Shukaku takes control of the sand-body to do with as he please until Gaara awakens, suppressing its 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's body. Gaara is killed in the process, though he is later revived by Chiyo to make up for the fact that she sealed Shukaku into his body in the first place. Gaara's ability to manipulate sand still appears to be intact, though to what degree he can use it remains unclear.
Yugito Nii/demon cat
Yugito Nii (二位 ユギト Nii Yugito?) is a Kumogakure ninja who had the Two-Tailed Demon Cat (ニ尾の猫俣 Nibi no Nekomata?) sealed within her. As the container of the demon cat her eyes are slanted and cat-like. Yugito can materialize the demon cat's form around her, giving her access to its fire-based abilities. Despite transforming into the demon cat to fight them, Yugito is captured by Akatsuki members Hidan and Kakuzu, though it is unknown which member was specifically assigned to capture her. The demon cat is later extracted from her body, resulting in her death.
Three Tails
The Three Tails (三尾 Sanbi?) is a gigantic, turtle-like demon that Akatsuki assigns Tobi (with the help of Deidara) to capture. Its abilities remain unclear, as it does little to attack its captors during its short appearance. It is clearly quite fast despite its size, as evidenced by its ability to keep up with Tobi. While the three-tails is distracted, Deidara molds an explosive clay fish in an attempt to capture it. It's unclear if this marked the end of the battle, as this is the last part of the battle seen before it is shown being towed away a short time later. It is later sealed by Akatsuki.
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.
Nine-Tailed Demon Fox
The Nine-Tailed Demon Fox (九尾の妖狐 Kyūbi no Yōkō?, English TV "Nine-Tailed Fox Spirit/Demon") is the most powerful of the tailed beasts. Its power is such that it is regarded as an age-old natural disaster, as a single swipe of one of its nine tails is enough to raise tsunamis and flatten mountains. Twelve years before the start of the series Madara Uchiha used his Mangekyo Sharingan to have the fox attack Konohagakure. None of the ninja of the village were able to defeat it until the Fourth Hokage, Minato Namikaze, arrived to deal with it. Minato sacrificed his life to seal the fox into the body of his newborn son, Naruto Uzumaki. Rather than completely seal off the fox's great power, however, Minato only sealed away a portion of it, giving Naruto access to the rest for unknown reasons. Minato left a key to Jiraiya that could either strengthen or weaken the fox's seal should the need arise. The seal naturally becomes weaker as the years pass, and in the event that it is broken the demon fox will be released.
The only initial indication that Naruto is the fox's host are the whisker marks on his face and his ability to rapidly heal from injuries and fatigue. Once learning of the fox's presence within him, Naruto makes better usage of its chakra by performing jutsu that would ordinarily be impossible for ninja of his age. Though small amounts of the fox's chakra constantly mix with Naruto's own, he can draw upon its reserves in two ways: he can enter his own subconscious and converse with the fox to ask for a donation of its strength. Alternatively, he can succumb to rage, allowing the fox's violent personality to influence his own. As Naruto draws upon greater amounts of its chakra the fox begins to manifest itself in the form of a fox-shaped red aura with multiple tails. Naruto can only maintain control over his mind for the first three tails; at the fourth, he loses all sense of self and readily attacks anyone nearby. In these instances, the fox can only be stopped by suppressing its influence, either with Yamato's Mokuton abilities or with a special seal created by Jiraiya that is directly applied to Naruto's body.
In the few instances where Naruto converses with the fox it is shown to have a loathing respect for both Naruto and Minato. While the fox initially complies to Naruto's requests out of curiosity, it later does so in an attempt to further its influence over Naruto. It also acts in order to preserve its own well-being, as Naruto's death would also result in its own.