Not exactly monotheistic; they only worship one god, but they believe in the existence of two-- Ahura Mazda (the good guy) and Ahriman (the bad guy).
Ahriman (the bad guy) or Aura Mainyu, is the origin of Satan, or The Devil.. ( That "other" bad guy )
And, IIRC, the concept of a Heaven and a Hell came from Zoroaster as well..
Later to be adopted by those "other" monotheistic religions..
I'm not so sure they actually worshipped Ahriman, as much as acknowledged his (it's? ) existence and influence on the lives of humanity..
The Good Guy