Killer B's Jinchūriki Forms

Killer B is the jinchūriki of the Eight-Tails, 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 other jinchūriki, he has full control of the Eight-Tails and his tailed forms. According to some Kumogakure shinobi who passed by and saw his full Eight-Tails form, the Fourth Raikage had forbidden him from fully transforming.

Version 1
Using his own chakra, Killer B can freely and quickly form a red demon cloak that surrounds his body, which he refers as. Once activated, his canine teeth and nails extend. The cloak itself gives Killer B 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. The attack power of this form is very high, Killer B can either decapitate a person with a single smash of his forehead, if his Lariat connects he can completely blow away their entire chest causing massive damage, and even destroy a massive platform where he trains.

He enters a three-tailed form during his fight with Taka; and one, four, seven, and eight-tailed forms 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 tailed forms, these tails are translucent with bubbles of chakra forming on the surface. Other than an incremental increase in Killer B'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 B can enter his forms. 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. To transform into Version 2, B forms eight Version 1 tails and wraps them around his body transforming him into Version 2, creating a crater underneath him. He can also enter Version 2 from his full Eight-Tails form.

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 B 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 B reverts to a version 1 form with the same number of tails. So far only the version 2 eight-tailed form has been seen, so it is unknown whether he can activate version 2 with fewer than eight tails.

Full Eight-Tails Form
While fighting Sasuke and his team, Killer B managed to wound Sasuke long enough to transform into the Eight-Tails' full form, in which his skin is destroyed in a similar fashion as 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 forms, he seems to be in full control of this form. He was forbidden to fully transform by his brother due to its immense power. However, B tends to ignore this order, as he transforms to train or to fight formidable foes, above other reasons.

In this form, he is able to create a Tailed Beast Ball in front of his mouth and releases it as a widespread blast, causing destruction on a large scale. In this form he is immensely durable, able to endure Amaterasu for a considerable amount of time-enough time to perform a Body Replacement Technique with a tentacle. He is also extremely fast in this form, able to block Amaterasu with his fist and strike extremely quickly with his tentacles.

In part with his control over the Eight-Tails, Killer B can transform parts of his body into parts of the Eight-Tails' body. Likewise, while fully transformed, Killer B can manifest his own human form from parts of the Eight-Tails' body.

Trivia

 * Unlike Naruto's jinchūriki forms or Gaara's Shield of Sand, Killer B's chakra cloak has not been known to attack or defend independently of its jinchūriki.