In the original teachings of Zoroaster, a Saosyant was any highly ethical, spiritually advanced person (“good men and leaders of the people”) devoted to teaching and transforming the world in preparation for Frashorkerti. 
In later teachings, this idea that many people could be Saoshyant was corrupted to the notion of a single saviour who would come and save the world. In this later tradition, which is an expression of the Chosen One archetypal corruption, three future saviors, one at the cusp of the last 3,000 years of the world, would be born. The last one born would lead humanity in the final battle against falsehood and deception, a.k.a. Frashokereti, or the final renovation of the world when evil is destroyed and humans lives happily ever after.
The Bahá'í tradition believes their founder, Bahá'u'lláh is the third Saoshyant.
- Boyce, Mary. “Ahura Mazda.” In Encyclopadia Iranica. New York: Columbia University, 2011.
- Boyce, Mary. A History of Zoroastrianism: Volume One The Early Period. New York: E. J. Brill, 1996. p. 235.