Board Thread:Episode Discussions/@comment-25319479-20160526105603/@comment-4912646-20160526154457

Ignoring Momoshiki, whose origins are a complete unknown factor, the only known requirement to awaken the Rinnegan is Hagoromo's chakra and (according to Kabuto's hypothesis) the Sharingan. The anime just took it one step further by establishing Mangekyō Sharingan as a requirement as well. So when Hagoromo awakened the Mangekyō, his chakra immediately combined with it and became the Rinnegan. The fact that he actually knew what a Mangekyō Sharingan was when his was the first and even called it by name certainly raises some questions though.