Not sure if this has been thought of already, but doesn't the gedo mazo thing that pain summons seem like a very nice fit for the body of the juubi? When it appears during this flashback we see it with a bandage over the eyes... or eye
And also all those things sticking out of its back, could those be the roots of tails? That might be a bit of a stretch because the shadow that we saw of the juubi
seems to show them all protruding from a common point, but i can't tell for sure by just looking at the shadow outline.