Mithra is the Iranian yazata of the covenant (Avestan mithra, 'contract'), whose great hymn is Yasht 10 of the Avesta: he stands between the parties to an agreement, sees every breach with his thousand ears and ten thousand eyes, and rides ahead of the sun in a chariot of gold. He is named with Ahura Mazda and Anahita in the inscriptions of Artaxerxes II, and his cult spread through Anatolia and Commagene - where his name is fused with Apollo-Helios - into the Roman world, where he became the Mithras of the mysteries. The Roman Mithras is a separate development with its own entry.
Modern Practice
In Zoroastrian practice he is honoured at Mehrgan, the feast of Mithra at the autumn equinox, and invoked as the witness of contracts, oaths and treaties; modern correspondence work also uses him for honest dealing and the protection of the sworn word.