I’ve been reading a doctoral thesis on Mithras and the beginnings of the mithraic cult in ancient Persia. Mithras was an old mazdean deity among indo-european tribes (aryans, particularly). During the migration process that split aryans into indians and iranians the cult of Mithras began to evolve differently, but it was mainly built around the concept of Mithras as the overseer of "contracts" - the keeper of order in the universe (the etymology of his name has been linked to both "contract" as well as "friend", but "friend" in the sense of "ally gained through a pact").
In the iranian tradition Mithras was a big figure that after the Zoroastrian reform was seen as sort of a right hand man to Ahura Mazda (in the Vedas, Mithras is conceived within the concept of a dvandva compound with his twin Varuna, and some authors have claimed that Ahura Mazda acts as a replacement for Varuna in a "iranian" dvandva). By then he had begun to be linked to the Sun (particularly the morning-midday time of the day), something that also stems from his more vedan days (Varuna is linked to the night/moon, and Mithras as his complementary is linked to the day/sun). His following was exclusively male, and there's evidence that since ancient times his cult was a mysteric one made up of mostly soldiers and aristocrats. Proof of that is the "mithraic" names that were so common among persian aristocracy around 500-400 BC.
This solar aspect was what made the greek authors later associate him with their Apollo. One particular image I saw on tumblr sort of made me think about this solar aspect of him, and because of other stuff I've been reading regarding Lucifer and its syncretism with pagan figures, I sort of got this image of promethean lucifer identifie with Mithras.