I'm bringing back my old theory about Pain being the host for remaining bijuu that Akatsuki has yet to sniff out. He may be either the 5 tailed beast, Gobi, or the 8 tailed beast, Hachibi.

五尾 | 彭侯 – Five Tails (Gobi), Houkou

Tale in the Legend of the Tailed Beasts: Houkou is a bijuu which looks like a five-tailed dog. Each tail of this creature represents one elemental power: Wind, Water, Earth, Lightning and Fire. They can cause catastrophic damage when used. Houkou will willingly destroy the nature to obtain the power he desires from these elements. He was once badly injured when he and Nekomata challenged Kyuubi. Strangely enough, he is the god of Illusion. Houkou lives in a giant, ancient tree.
Situation in the Ancient War of the 9 Gods: Battled 4 times; 3 Wins, 1 Loss
Wins: Sokou, Nekomata, Raijuu
Losses: Kyuubi
Fate: After receiving a severe wound, Houkou runs away to one of Japan's islands, and hides in a place near Kyushu’s Volcano - a giant tree, to heal his wound. But the tree's concrete position is not known by anyone.
Japanese Myth Appearance: A species of dog, with 5 long tails. Has a white coloured body. Each of the tails possess one of the five elemental powers, that used simultaneously can cause a big earthquake.
Ability: Attacking using mixture of elements; Houkou is skilled in each kind of element ability, and can also create instantaneously each kind of natural disaster.
Bijuu Strength Ranking: 3rd
Bijuu Chakra/Stamina Ranking: 5th
Symbolic Element: Illusion (Being the god of Illusions)
Origin / Discovered in: Near one of Kyushu’s volcanoes (Akase Lava Volcano, probably)


八尾 | 八岐大蛇 - Eight Tails (Hachibi), Yamata no Orochi
(Also known as Hachimata)

Tale in the Legend of the Tailed Beasts: Yamata no Orochi is a Snake bijuu. It has crimsom red eyes, eight heads and eight tails (also depicted to having trees and vines growing around them) and has the power of the Demon world, a symbol of evil. Each head of Yamata no Orochi represents a symbol: Soul, Ghost, Evil, Devil, Monstrous, Kill, The Afterworld and Death.
Its powers were originally weak, the members of the Kusanagi clan, when attacking it, acted carelessly, using the legendary sword Kusanagi no Tsurugi (Oro’s sword). Because of that, as a consequence, Yamata no Orochi takes the sword from them, absorbing the power contained inside, and becoming an incredibly powerful creature, and finally storing it in its body.
With the new power, Yamata no Orochi releases an insane amount of dark energy, awakening the other Bijuu and becoming arrogant. Yamata no Orochi proceeds to defeat many of the Bijuu, but when it battles with Kyuubi, it is defeated. The cause of this is simple: Kyuubi has an unlimited amount of power, making even a hole in the Kusanagi sword. As this happens, it shows how tremendously powerful Kyuubi is, deserving of the “King of Bijuu” title. Not even Sarutobi and Enma (I’m not sure how those two are into the legend) could scratch the Kusanagi no Tsurugi. (That makes connections to the Oro x Sandaime battle)
Legend also depicted it as having a power able to rival Kyuubi’s [in quality], but the fact is, Yamata no Orochi’s does have a limit.
Situation in the Ancient War of the 9 Gods: Battled 5 times; 4 Wins, 1 Losses
Wins: Shukaku, Isonade, Kaku, Raijuu
Losses: Kyuubi
Fate: Yamata no Orochi awaits a genius of the Yagami (NOT the Kanji of Yagami from Death Note, but Yagami as in Eight Gods: 八神) clan, which is the arch enemy of the Kusanagi clan. The purpose is to awake it [Yamata no Orochi], in order to fight Kyuubi again.
Since Yamata no Orochi’s power inherits in the Yagami Clan, If the heir appears as the first genius in 1000 years that is able to awake Yamata no Orochi’s ability, it will be completely restored. Once the process is done, the genius with such crucial task will be destroyed due to the true body of Yamata no Orochi resurfacing, which has an all-new power. It won’t be found before the decisive battle with Kyuubi, and will be able to kill no matter of thing, destroy any type of place, and attain infinite vision (as Kyuubi has). The core of Yamata no Orochi’s power is in a kind of forbidden sealing jutsu. As soon as the Yagami clan heir comprehends enough of it, a bigger, much stronger power will surface in the grasp of Yamata no Orochi.
Japanese Myth Appearance: A super Snake of Eight heads and Eight tails, attached to a single body. Has a deep brown color. It definitely is the biggest bijuu - the lenghth of it is over eight mountains. Originally, it was weaker than Houkou and Nekomata; however, that was 1000 years before a brave warrior tried defeating it with the Kusanagi no Tsurugi. In the process, the sword’s power is absorbed instead by Yamata no Orochi, thus greatly expanding his power. He continues though to be able to use freely the sword in his belly.
Ability: The legendary sword: Kusanagi no Tsurugi; summoning of evil spirits
Bijuu Strength Ranking: 2nd
Bijuu Chakra/Stamina Ranking: 2nd
Symbolic Element: Devil
Origin / Discovered in: An old battlefield in Osaka
Personal notes: Now, everyone knows this one resembles Orochimaru, but what I like is the “Yagami genius” legend, which OF COURSE screams Sasuke.
More info about Yamata no Orochi:
There is the story of the killing of the Eight-Headed Serpent by Susanoo. According to the legend, a local town had been living in fear of the Serpent, and the townspeople offered the sacrifice of a young girl each year to keep it appeased. Lord Susano heard about the town's dilemma, and became determined to slay the beast. He exterminated the serpent by serving it alcohol, and then cutting off its heads while it was intoxicated. Between the Kagura performances, this one is the most famous.


(Taken from a thread posted by K-oz - http://www.mangashare.com/forums/nar...ed-naruto.html )


If Kishi was influenced alot by the legend of the tailed beasts then he probably made the bijuu in the manga as power as they are said to be in the myth. Meaning Kyuubi is the strongest, Hachibi being second, and Gobi being third. I say Pain could be the jinchuuriki for the 5 tailed beast cause his ability to control the weather with jutsu(so it seems) Its said that Gobi is pretty well skilled in all the elements. Also now that we can assume that Pain may have more then one body we can also assume that he has different abilities and jutsus that he uses with these bodies. To me it would suit the power of the 5 tailed beast, Gobi. But if Gobi was already captured then he may be the 8 tailed beast Hachibi. Going by the legend you would think Hachibi would be more suited for Orochimaru but with the rumors going around about Pain and how powerful he is, I thought that he may be the container for the 8 tailed bijuu since Hachibi is considered to be the second strongest bijuu in the legend.

This is all just speculation and assuming that Kishi took great interest in chinese mythology and the legend of the bijuu.