Hello NarutoBase, this is U C H I H A. Today I will be showing off my first thread but more specifically a theory. Please keep in mind this is my first thread; I apologize for any formatting errors etc. Thank You.
~ U C H I H A
Rikudou Sennin is a Member of the Juugo Clan
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The Sage of Six Paths; the God of Shinobi, the most powerful ninja, the only man who can defeat the Juubi. He was given a countless number of titles and is undoubtly the most powerful ninja ever. However how can we assume he is from the Jugo clan - it doesn't have a name!
However we know that the clan has the ability to absorb Natural Energy. This is such an underrated power and makes the clan too OP had it not been for the surge of Sage Transformations the clan members have due to not being able to control the energy. Keep this in mind.
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The Juubi is the source of all chakra and we learned this and this is quite an overlooked fact. Now look back all the way to the beginning of Naruto - what are the Five Basic Ninjutsu Transformation:
Lightning Release, Fire Release, Water Release, Earth Release and Wind Release
What do these all have in common? They're all aspects of nature! The Juubi, is the manifestation of nature itself. It explains its violent behavior and lack of morality and simply a mind at all - nature doesn't have a goal or morals; it does what it needs to do to thrive and this is why Juubi seems so hostile to the humans as they're natures greatest threat. So now we have concluded: Juubi = Nature "Personified"
Back got the Sage - why of all names is he called the 'Sage' of Six Paths - simple, he is a Sage. Now, he mastered Sage Mode - beyond perfection. He was a human who in a way mastered nature. Together, in an essence he was the ultimate being, possessing the perfection of a human AND nature. This is why he defeated the Juubi - Nature + Human > Nature.
From the the Juubi he obtained the Rinnegan - the perfect human now with even more perfection that is nature awakened the eyes, that being Rinnegan. This is why the Gedo Mazo was made of Wood which absorbs energy - because the Juubi lost all it's internal power it is gathering outside power. This is why Wood Release absorbs energy - because it is a descendant of the Sage and the Juubi himself.
Here's a bit more evidence - Kabuto said he is the closest thing to the Sage ... Ever. This is because he mastered natural energy with Juugo DNA that's why. Ma and Pa, the two frogs said Sage Mode is the most powerful a human can achieve alone. Sage Mode = Natural Energy + Physical Energy + Spiritual Energy.
The Jugo kept the Natural Energy, the Senju got the physical energy and the Uchiha got the spiritual energy. Now I know somebody would say - the Sage is an Uzumaki because they're good with fuinjutsu. But think about this - if the Sage had the Juubi in him and the had kids, why can't the Uzumaki perform ninjutsu? Think... Before you say Naruto he is only half and before you say Nagato he had the Rinnegan. Why can't the clan with the most perfect ninja use jutsu? Because he isn't a member of the clan.
Also, the Sage of Six Paths had horns, so did Kabuto who mastered Sage mode with Juugo clan DNA. Now another arguement is why does Kyuubi Mode Naruto look like the Sage - because the Kyuubi is part of the Juubi and Naruto mastered Sage Mode as well. That's why. Also this is why the Sage said, the Bjuu will one day have to comeback together. As Derp Obito said, there is a heavy triad kind philosophy. Three pupils per team, three tomtoes etc. Natural Energy, Physical Energy, Spritual Energy - they can't thrive alone - one day the Bjuu must come back together because without the Natural Part, peace can't be attained!
Hope you liked the theory! Debate, argue and have fun but no profanity please! 
P.S. This was done on a phone so sorry for an mini problems. Thanks!
-The sage had horns??!! Wow....
-He's called the So6P because there are 6 main paths, with the exception of the Outer path which lets the Rinnegan user control life and death.
-It doesn't have a consciousness which is why it lacks morality. And does things indiscriminately.
-Gedo Mazo is not made of wood. Gedo Mazo is the husk of the juubi.
-And who says that the Uzumaki's can't perform ninjutsu? They're just better at fuuinjutsu.