It had the Rinnegan back when the Sage of the Six Paths was it's Jinchuriki, and you know how the Mangekyou Sharingan has the ability to control tailed beasts? That is why it's eye is a Sharingan now, because Madara is controlling it.
it's still unknown, we have lots of people who says that the rinnegan came from juubi, or the opposite, that sharingan came from him or something, or maybe dojutsu came from his eyes
It's clearly not a regular rinnegan. It's red and has 10 tomoe in it. Possibly the evolution form of rinnegan or a whole new dojitsu. Kishi will elaborate on this hopefully.