Alot of people are saying that Sora is the eight tailed beast, as in the manga it says that there are only two tailed beasts left to be captured, the nine-tails and the eight-tails. I also believe that he is the eight-tailed beast.
the anime seems to have jumbled up a few things in the manga...
you're actually looking too far ahead there... hidan and kakuzu capture one which was a ninja from the cloud village (the two tails) in 313 and it hasn't shown tobi and deidara catch the 3 tails in the lake thats in manga 317-318
in the anime... there are 3 tailed beasts captured including gaara's shukaku... then there is the cloud village nin with 2 tails and then there is the 3 tails... and obviously he can't be the nine tails.
that leaves you with 3 left out there which he could be including the 8 tails.