A digest-sized magazine from France, CINEMA 57, has frequently been cited as being the first-ever monster magazine. I disagree. True, it is a magazine that has monsters in it, but it was not ever meant to be an on-going periodical, which I believe is the qualifier. Warren's test run of the soon-to-follow FAMOUS MONSTERS OF FILMLAND in AFTER DARK #4, while content-heavy with monsters, can be similarly categorized.
Nope -- unless a hitherto unknown moldering tome is miraculously resurrected from the trash heaps of time, for me, FAMOUS MONSTERS OF FILMLAND is the first, regularly published monster 'zine.
However, that doesn't stop dealers from fetching premium collector's dollars for CINEMA 57. Two sellers are offering the 145 page, July 1957 French publication on eBay. One copy is described in "very good" condition and is selling for $458. The other is being sold as a "good" copy for $249.