In mythology, myths were traditional stories telling of ancestors, heroes, or supernatural beings, that were prominent in revealing the world view of a particular culture. Many myths were based on truth. (TOS: "The Way to Eden")
A Tal Shiar myth claimed that the organization was merely a mask worn by a far older cabal called the Zhat Vash. (PIC: "Maps and Legends")
Until the Aenar were discovered in the early 22nd century, they were considered a myth by the Andorians. (ENT: "The Aenar")
The existence of the SS Botany Bay was held to be a myth by Arik Soong. (ENT: "The Augments")
Both katras and the Kir'Shara were thought to be myths by most Vulcans until the Vulcan Reformation in 2154. (ENT: "Awakening", "Kir'Shara")
Lieutenant Carolyn Palamas served as the archaeology and anthropology officer aboard the USS Enterprise. Her particular specialties included "ancient civilizations, relics, and myths." (TOS: "Who Mourns for Adonais?")
According to Jean-Luc Picard, China was thought to be a myth until Marco Polo found it. Similarly, Wesley Crusher was informed falsely that the Iconians were only a myth. Picard compared the two situations in 2365 after the USS Enterprise-D located the Iconian homeworld. (TNG: "Contagion")
On the planet Rakhar, the Rakhari told stories about changelings. Croden believed them to be myths until he met Odo in 2369. (DS9: "Vortex")
According to Kathryn Janeway, "Federation cosmologists had a theory that the Omega molecule once existed in nature for an infinitesimal period of time at the exact moment of the Big Bang. Some claimed Omega was the primal source of energy for the explosion that began our universe." Seven of Nine, however, felt that that was "a creation myth, like any other." (VOY: "The Omega Directive")