Board Thread:Naruto Discussions/@comment-3568954-20140625232535/@comment-24891350-20140626043508

The third eye is the evolution of the normal Rinnegan which would also make it an evolution of the Sharingan that's why when Hagoromo said that Kaguya had the power of the Sharingan, he was probably referring to Sharingan and Rinnegan. However since the third eye is closer to the Rinnegan than it is to the Sharingan everyone calls it a Rinnegan.