There is a theory I have been thinking about lately,regarding the rinnegan.How did Nagato get his rinnegan?Was he born with those eyes.Lately I thought that he wasnt.In fact I thought these eyes were implanted in him(by who?Madara?)
As for the mighty Rikoudu again I think he wasnt born with the rinnegan too.Maybe he got the rinnegan after he had sealed the Jubi thus obtained its power.This theory is based on facts:
+We dont see Rikoudus face when he fought Jubi so we dont know if he already had the rinnegan before he sealed the Jubi.
+The design of Jubis eye is like the combination of rinnegan and sharingan.I dont think the similarity of Jubis eye and the rinnegan or sharingan is just by accident.

but to defeat the juubi you need something very powerful like a kekkei genkai like the rinnegan and i think you don't get them when your born i think they are like the sharingan and you awaken them

when I said someone was born with the rinnegan or sharingan I meant those abilities were in their blood already they just had to awaken them sometime in their life.
Well Yes one needs a very powerful source of power in order to defeat the Jubi but not neccesarily the rinnegan,or some other crazzy kekkei genkai.A lot of powerful shinobis in the history dont possess any kind of kekkei genkai:Hiruzen,Tobirama,Minato,Hanzo,Sakum­o,R­aik­age­,et­c.

yes thats true

Ok so this is my new theory,completed version:
Before he defeated Jubi Rikoudu didnt have the rinnegan.The only source of power he had was the 'body'-the physical ability.As for the Jubi,it possessed a special eye,which was a combination of rinnegan and sharingan(with the rinnegan was the 'good half' and the sharingan the 'evil half').After Rikoudu defeated Jubi he sealed the demon inside himself,thus gaining the power of its eye.Since the Sage was 'good'-he wanted to bring peace to the world-the power of Jubis eye,while used by him,was shown as the rinnegan-the 'good' part.Before he died he gave his younger son the power of his own-the 'body' and gave the older son the power that he had obtained from the Jubi-the 'eyes'(by some mean,maybe by some jutsu).However,time passed and the heirs of the older become more and more possessed by hatred,thus their eyes turned into the 'evil half'of Jubis eyes-the sharingan.
Turning to the story about Pein.After losing the battle against Hashirama,Madara began to doubt if the power of even the 'Eternal' Mangekyo sharingan was enough to defeat the Senju,and he spent years to look for an even more powerful source of power.He then discovered the tablet that Rikoudu left behind,and using his sharingan he was able to unlock some parts,and discovered the secret of the Uchiha clans eyes-the sharingan,with the power of the rinnegan hidden within.After some experiments he finally succeeded in turning the sharingan into its 'good' half-the rinnegan and then decided to experiment this on someone-a child named Nagato.And thats the story how Nagato had the rinnegan.
Ok thats my theory,the parts about good and evil were added just to make things sound more like a fairy tale lol,plus I couldnt find a way to put the byakyugan into the story.

wow very interesting you put a lot of thought into this and it makes sense

thanks

Its a pretty good theory nothing can clearly disapprove it. We can just wiat and see i guess if it does turn out your right though lol that would be amazing to say the least and would be a huge turn of events.