Already been theorized plenty of times.
The sage supposedly conquered the juubi with his massive chakra/life force and his trusty arsenal of weaponry only manageable by his chakra surplus and sturdy body. It wasn't until becoming the jin that he awakened the rinnegan as an outward manifestation of his hosting of the juubi.
^In a nut shell. I support it anyway though. I feel as though we will come to learn more of Rikudou as we get closer to the end of the series, perhaps even getting his own flashback or something along those lines. We thought Hashi was a stern stone cold SOB before we he recently got edo'd. I bet Rikudou was a goof too. Kishi's going to troll hard.
