When did local myths become a part of the puranic stories
Answers
Puranas means "ancient" and it's a vast genre of Indian literature about wide range of topics particularly myths, legends and other traditional lore.
Composed in Sanskrit but in regional languages several of these texts are called after hindu deities as Vishnu, Shiva and Devi.
Puranas genre of literature is found in Hinduism and Jainism both.
When men discovered the presence of Puranas in the society, they were curious enough to know what is being implied by such lengthy documents of knowledge.
Based upon the knowledge imparted by the documents, those men started to believe in some supernatural occurring to them as well, and add that portion to local chats as well, which later transformed into several other texts as well, known as mythical texts.
Here, the mix-up was made.