From the very beginning of Iranian disciplinary studies, the material concerning Zarathustra’s biography has been analysed in depth, firstly to identify the homeland of the Prophet and then to discuss the historical reality of this authoritative figure. Despite the divergences of opinion, emphasis has always been placed on the reconstruction of the figure of Zarathustra and much less on the socio-cultural context in which the image of the Prophet was cultivated. The present paper represents the first part of a larger work that aims to reverse this perspective and emphasise those data, which link up narrative variations and extensions with local identities. In fact, variations in geographical setting reveal processes of acculturation through which social groups reinvented the influential image of the Prophet within a familiar horizon. In this respect, the Sasanian period proved pivotal in the formation of both Zoroastrian and Iranian communal identities, while this first work will analyses aspects connected to East Iran and the Khorāsān tradition.