Just a thought...
Kabuto seems to be hell bent on becoming just like the sage of six paths. He even mentioned that he was closer to the sage then anyone else...http://www.mangakong.com/manga/Naruto/582/5
I was wondering how the sage could have defeated the juubi before he had the RG. I assumed he was Uzumaki thus capable fo powerful seals to just stop the juubi and seal it away. However, that still doesn't seem enough. Judging by the shadowy figure and the resemblance to juugo. Perhaps RS was originally a dragon sage which is how he got the horns and tomoe.
Juugo from the side (notice the hair texture and large nose)http://www.mangakong.com/manga/Naruto/579/12
Rikudou sage from the side..(notice his hair is the same texture, but longer and the noses are the same as well)http://www.mangakong.com/manga/Naruto/572/14
And don't give me the "so they look alike, allot of characters do" argument when descendants of Uzumaki are marked by red hair. The same nose and hair texture could be enough, plus the horns too.
Look at the ryuuchudou picture, there are tomoe flames...http://www.mangakong.com/manga/Naruto/579/14
We know the RG is the ultimate doujutsu with 3 circles on the pupil, but no Tomoe. Perhaps the Tomoe and they're use came from RS's use of Ryuuchidou sage power's meaning the uchiha's fuuin on they're pupil for the SG originates from Ryuuchidou and why certain uchiha have special bodies like madara and apparently sasuke.
And the pattern around kabuto and Orochimaru's eyes is similar to a tomoe, but is likely a black pigmentation like naruto's due to the use of the Ryuuchudous natural energy IMO...http://www.mangakong.com/manga/Naruto/582/5
Perhaps this design just expanded on the RS to his chest with the six tomoe pattern.
And for some reason the Kyuubi Kuurama has the same tomoe-like design around his eyes too... It extends from his ear though. This may have nothing to do with the use of sage chakra in the kyuubis creation. But, allot of the bijuu have dark circles around they're eyes too (those in which u can make out they're eye, that is). We will see...
Just a thought at this point...