Don't think there is a bus. I vote a dragon or whomping willow. No multiple arms.
One other possible idea though... what if that cylinder that Teebin drew was a moving screen on an arm and we were in the center of it? Just a thought.
You're right, it can't be the bus. How could the Knight Bus get onto the Hogwarts grounds or navigate the Forbidden Forest? It's not screens though, as Teebin said a couple of pages back that he THOUGHT they would do this effect with screens, but after hearing this new insight from his source, he was proven wrong, and that they would be using something that had to do with the "angler fish" (KUKA ARMS!).