Here's a web site you might enjoy: The Eggcorn Database

The term "eggcorn" was coined by someone who saw someone else use the spelling "eggcorn" for "acorn."

He was struck by how oddly appropriate it was. An acorn is sort of egg-shaped. For someone who didn't have a clue how to spell "acorn," "eggcorn" was a reasonable guess. "Eggcorn" implies a sort of false etymology. You could call it "folk etymology" if enough people used it, but this was probably one individual's mistake.

You could say a mondegreen is a type of eggcorn. Mondegreens pertain exclusively to song lyrics, whereas eggcorns could occur anywhere in spoken or written language.