The term folktale refers to a story passed down from one generation to the next.
I suggest the creation of a new term: folkfact.
A folkfact is a fact that is handed down through generations. The accuracy of a folkfact is often not questioned, and its origin is unclear.
Rusty objects cause tetanus
Mongol warriors, knowns as Tartars, invented steak tartare by tenderising meat beneath their saddles.
Folkfacts are similar to common misconceptions, for which Wikipedia provides a comprehensive list.