A Big Happy Family

X wrote the novels -Tarzan Alive and Doc Savage: His Apocalyptic Life – which were sort of fictional biographies. In them he hypothesised, that the real meteorite (Y meteorite) which fell near Y, Yorkshire, England, on December 13, 1795, was radioactive and caused genetic mutations in the occupants of a passing coach. Many of their descendants were thus endowed with extremely high intelligence and strength, as well as an exceptional capacity and drive to perform good or evil deeds. The progeny of these travellers were purported to have been the real-life originals of fictionalised characters, both heroic and villainous, over the last few hundred years, such as Sherlock Holmes, Tarzan, Doc Savage, and Lord Peter Wimsey. This created the Y universe which was later expanded by X and other writers to include The Scarlet Pimpernel, Sherlock Holmes, The Spider,James Bond, Nero Wolfe, Sam Spade, The Lone Ranger, The Green Hornet, and even Star Trek.
 
This concept has been used as a unifying device by others such as Warren Ellis’s (Planetary), Alan Moore (The League of Extraordinary Gentlemen) and in Tales of the Shadowmen edited by Jean-Marc Lofficier, using characters from French Literature.
 
X and Y please. 

Epunymous

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?