It was said that the Eigh-Tails have always gone loose from its jinchuriki until it finally found Killer Bee. During this time the Third Raikage always had to surpress it with the Amber Purifying Pot temporarily until they find a new jinchuriki. The Eight-Tails is also the only bijuu to get along with its host, completely controlling it. Killer Bee did this with his training in the Falls of Truth.
The Eight-Tails was first introduced when Taka attempted to capture it in Part II. It managed to trick them to believing they had captured him and gotten away. It was seen again when Kisame attempted to capture it as well and failed.
Though it still maintained animal like personality, ever since Killer Bee gained complete control over it, their relationship has been more like friends. The Eight-Tails is shown to have a logical mind sometimes helping Bee at making decisions. Though it is smarter than most of the bijuus it still sometimes have a bad temper.
The Eight-Tails is supposed to have the highest amount of chakra besides the Kyubi. It has a humongous stature with brute strength. It's octopus like being also makes it have the ability to produce ink to blind its opponent. It is capable of using the Tailed-Beast Ball as well.