You searched for: “allonyms
allonym (s) (noun), allonyms (pl)
1. The name of another person, especially that of a significant historical figure, which is assumed by a writer: Examples include Mark Twain that was an allonym for Samuel Clemens and George Eliot, the allonym for Mary Ann Evans.
2. A book bearing the name of someone other than its author: An allonym can be a published work that uses the name of an individual different from the author.