I have to correct this. “Folk etymology” is a specific term in linguistics to describe how words change from an unfamiliar pronunciation to one more familiar to the speaker.
One example is “turtle.” The French word for the shelled lizard was “tortue.” The word “turtle” already existed in English as the name of a bird (for the sound it made). English speakers had trouble pronouncing “tortue,” so, over time, started using the the word “turtle,” which is close. The bird took on the name “turtledove” to differentiate.
“Isinglass” is another example, changed by folk etymology from the Dutch “huizenblaas.” There’s also the Picketwire Canyon in Colorado, originally named “Purgatoire” (purgatory) by French speakers.
What we’re talking about in this thread are etymological urban legends.