Topic on Forum:Trope Talk
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"An Aesop" is because "Aesop" would be confused with the person responsible for Aesop's Tales.
But the only reason that confusion would happen in the first place is that we're using the term "Aesop" instead of "Moral" or "Lesson" or similar. Given that we already have an English word for it, why use "Aesop" in the first place?
(Of course, "Moral" would result in its own confusion--ironically, if we shifted to that term we may need a 'The' on it to indicate that it is a noun and not an adjective.)