protomyth

noun1

1. •
   An initial form of a myth that is inferred or known about only from secondary sources; a motif that is not yet accorded the status of myth, but may (it is supposed) become one.

   • Petrosyan, noting that this story links the water-horse to the twins' conception, proposes a protomyth in which the horse is the twins' father. — 2005