Board Thread:Theories and Speculation/@comment-29033274-20160713180744/@comment-1512913-20160714005104

Just like Chidori/Raikiri are similar to whatever technique 3rd Raikage used. As our friend NeedleJizo has stated, the conception of the Rasengan came to be when Minato observed a Tailed Beast Ball. On top of that, the Rasengan was said to be the highest level of shape transformation, while Asura's Wind Rasengan-like orb clearly isn't well shaped or stable.

But sure... for all intends and purposes, Asura used a prototype 'Rasengan' but not really an actual one.