Killer B's Jinchūriki Forms

Killer Bee is the jinchūriki of the Eight-Tailed Giant Ox, and is therefore capable of transforming into various forms that resemble his tailed beast, until he finally takes on the complete form of the Eight-Tails. So far, he has displayed a greater variety of forms than any other jinchūriki. Unlike Naruto and Gaara, he has full control of the Eight-Tails and his transformations. According to some Kumogakure shinobi who passed by and saw his full transformation, the Fourth Raikage had forbidden him from fully transforming.

Version 1
Using his own chakra, Killer Bee can freely and quickly form a red demon cloak that surrounds his body. Once activated, his canine teeth and nails grow long and sharp. The cloak itself gives Killer Bee paws of chakra around his hands and horns on his head. He can also create an additional set of horns on his forearms to use for his Lariat attack.

He enters a three-tailed form during his fight with Taka, and one, four, seven, and eight-tailed transformations at different points during his battle with Kisame. Kisame's Samehada is able to absorb up to six of these tails at a time. Like Naruto's first three transformations, these tails are transparent with bubbles of chakra forming on the surface. Other than an incremental increase in Killer Bee's strength and speed, there are no obvious differences between the number of tails.

Version 2
By tapping into the Eight-Tails' chakra reserves, Killer Bee can enter "version 2" transformations. As stated by Sabu, this is done by forming the chakra into a human shape. These transformations are meant to give him an edge in battle without completely releasing the Eight Tails, as that would endanger anyone around him.

The version 2 chakra cloak is much darker than version 1 (similar to Naruto's four-tailed form and upwards). Hooves form around his feet, the horns on his head elongate, and a final "chin horn" resembling that of the Eight-tails appears. While in this form Bee becomes more muscular, and if necessary can create chakra bones, such as a bull's skull, to increase the strength of his attacks.

When struck by Samehada, only the Eight-Tails' chakra is absorbed, so Killer Bee reverts to a version 1 form with the same number of tails. So far only the seven and eight-tailed version 2 forms have been seen, so it is unknown whether he can activate version 2 with fewer than seven tails.

Full Eight-Tailed Transformation
While fighting Sasuke and his team, Killer Bee managed to wound Sasuke long enough to transform into the Eight-Tails' full form, in which his skin is destroyed in a similar fashion of Naruto when he reaches the fourth tail and above. In this form, he takes on the appearance of the Eight-Tails itself, rather than just a cloak, taking on the appearance of a huge ox-like creature with eight octopus tentacles as its tails, and four horns on his head, one of which is broken. Like his previous transformations, he seems to be in full control of this form, though to what extent is still unknown. He was forbidden to fully transform by his brother due to its immense power. This, however, didn't prevent Killer Bee from doing so in desperate situations.

In this form, he is able to create a huge sphere of chakra in front of his mouth similar to Naruto's Four-Tailed Fox Menacing Ball. Instead of a concentrated ball of chakra, he releases it as a widespread blast causing destruction on a large scale. However, even in his full form, he does not possess an near-invincible shroud like the Nine-Tails, so even with his immense size, powerful attacks such as Chidori Sharp Spear and Amaterasu, prove effective against him.

In part with his control over the Eight-Tails, Killer Bee can turn parts of his body into parts of the Eight-Tails body. For example, he can turn his chakra tails into a solid Eight-Tails tail. Likewise, while fully transformed, Killer Bee can manifest his own human form from parts of the Eight-Tails' body.

Trivia

 * At one point, Sabu is shown saying Killer Bee had entered a Version 1 seven-tailed state, despite the fact that eight tails are shown.
 * Unlike Naruto's jinchūriki forms or Gaara's Shield of Sand, Killer Bee's chakra cloak does not attack or defend independently of its jinchūriki.