Where did legends originate?

Myth - a concocted story made up to explain the unknown.

Legend - a traditional story held to be true

History - an attempt at converting available evidence into a coherent story.

A legend is a traditional story, held to be true. A good example is The Iliad, which originated with the strolling bards, who sang it as they plied their trade in Greece. Each had their own version(s) and it was not until the emergence of alphabetic writing there in the 8th Century BCE that one version was written down (attributed to Homer). Even that version had been rewritten several times before it came down to us today.

So no version of a legend is 'true', any more than is 'history' of which there are many versions, according to the preferences of authors and readers. We believe what it suits us to believe, or keep and open mind and look for the best logical answers.