In the mythology of Yasht 8.21-29, Tishtrya, as a mighty white horse with golden ears and golden tail, rushes towards the cosmic sea Vourukhasha. On his way, he is confronted by Apaosha as a horrible black horse with black ears and black tail. They battle for three days and nights until Apaosha drives Tishtrya away.
Tishtrya then complains to Ahura Mazda that he was weakened because humankind did not give him his due of proper prayers and sacrifices.
Tishtrya then causes the rains to fall freely upon the earth and all is well again.
This legend has been interpreted to be a mythological conflation of a seasonal and astronomical event: The heliacal rising of Sirius (with which Tishtrya is associated) occurred in July,[a] just before the hottest and driest time of the year.
Ahura Mazda then himself offers sacrifice to Tishtrya, who now strengthened reengages Apaosha in battle at noon and conquers the demon of drought.