I have to admit, I've wondered this for a long time: Why do we have this phrase? The words "moral" (for a story's intended lesson for the reader) and "fable" (a fantastical story meant to convey a moral - precisely what Aesop was known for) and "parable" (the same thing for non-fantastical) already exist. "An Aesop" is LESS clear, since it could mean a moral, a fable, or a parable. You need context to figure out which. It's also redundant, since clearer terms already exist. It seems like a lingual construct as pointless as adding "head of" to quantities of cattle. I'm not trying to agitate for its removal, mind - the term and the article itself are both probably too widely spread to excise. I'm more just looking for justification for the sake of my own peace of mind.