"...some underappreciated artist somewhere who thought up the mushroom idea."

It's an ancient trope involving fairies and the fey, gnomes in particular. I myself used it in the late 1970s when I decorated the inside back cover of a math book in Elementary Algebra I with a drawing of an elf warrior standing on a mushroom holding his sword upright overhead where it was struck by a beam of force from an unknown source. My math teacher, Mr. Smircich, loved it. Gave me an A in the class.

Thanks!