In the religion known as Zoroastrianism, Ahriman is the equivalent of the devil. He fought against Spenta Mainya (the Holy Spirit) and Ahura Mazdah, the supreme "Father" god. Furthermore, Ahriman tried to hurt humankind.  Ahriman introduced the frost in the winter, heat in the summer, and diseases throughout the year. He also created Azhi Dahaka, a dragon who could bring ruin to the Earth. Azhi Dahaka created the planets against the stars as he sprung up to heaven.
Ahriman is considered to be the embodiment of evil (darkness, etc). When he invaded Heaven, it was in the form of a dragon. However, Mithra battled and won against Ahriman. It is also said that Ahriman had dragon minions.