I thought they were monotheists.
That is how they are generally counted, but they are sometimes classed as a dualistic religion, because their devil, Ahriman, was nearly as powerful as their God, Ahura Mazda. It is thought that the powerful Chrisian Manichaean heresy was derived, from ideas of the Zoroastrians. The Manichaeans believed that the world was evil, and under the dominion of the devil, instead of being the providential gift of God, in which evil is freely chosen by men. Some of these ideas can be perceived in a few Christian religions even today.