The problem of interpreting one of the most mysterious images in the mythology of the world is solved. It is the interpretation of the image of the three-legged donkey contained in Zoroastrian texts. Existing interpretations do not explain all the elements of this image and their ontological nature. The attempt of revealing the specificity of the language of myths which consists largely of a special kind of symbols has been made. It is a kind of symbols of increased social significance. It is shown how the process of sacralization the life world of ancient Iranians has been carried out with the help of those symbols. Ancient Iranians were people who created Avesta and other sacred texts of the Zoroastrian religion. Instead of cosmological and other approaches of naturalistic character widespread in the Iranian studies, a socio-cultural approach is applied. The tropological approach is also used, i.e. the view on the image of the main character of the myth as the tropes that makes sacred sense in the life of the society. A new interpretation of the sacral donkey whose image symbolizes the essential characteristics of the Zoroastrian faith.