Science Fiction author Reginald Bretnor using the pen name Grendel Briarton wrote a multi-year series “Through Time and Space with Ferdinand X!”, in which each installment was a short-short that ended in a horrific pun. X and the nature of the stories—detailed and tedious, yet ending in vaguely familiar catchphrases—may have been inspired by Walter Bagehot, a major literary and political figure from the late 1800s now fallen into obscurity.
X is now used to describe stories of this kind. Isaac Asimov was particularly notorious for these.
An example Death of a Foy
What is X?