Petraios along with the eleven other rustic daimones had been appointed guardians for the future Olympian Dionysius while the Nysiades took care of the baby. When Hera learned that the Daimones had been protecting the baby she turned them into ox-horned centaurs. Despite his change Petraios and the other Daimones change they still stayed loyal and were with him when he made war in India. They were among the first of his followers.
The Nymphai Lamides
Originally he and eleven other centaurs were rustic daimones (spirits). However, when Hera learned that they were assisting in the protection of Dionysos, she transformed the twelve into beastly ox-horned Kentauroi (Centaurs).