Greek mythos originally meant truthful powerful speech, not fiction.

Evidence

In his book Theorizing Myth, Bruce Lincoln opens with a fascinating early history of the Greek terms mythos and logos. Thus we are told that Hesiod’s Works and Days associates the speech of mythos with truth (alethea) and the speech of logos with lies and dissimulation. Mythos is powerful speech, the speech of heroes accustomed to prevail. In Homer, Lincoln points out, logos refers to speech that is usually designed to placate someone and aimed at dissuading warriors from combat. — Talal Asad, Formations of the Secular, 109

Categorization

• DEFINE: The etymological reversal: mythos (not logos) was the Greek word for speech associated with truth; logos originally meant placating, dissuading speech.
• XREF: Reverses the modern conflation where ‘myth’ means falsehood; connects to the reader’s broader interest in how words’ meanings shift with power.