funny story, Almost all mangaka originally began as Doujinka (fan comic artists) hell, before they got famous for their original Manga, CLAMP famously made a Jojo's Bizzare adventure Doujin where the male characters fucked and got eachother pregnant
and many mangaka continue making Doujin, or even Hentai, on the side under pseudonyms....I mean the fact that Mashima's artstyle is Identical to Eichiro Oda's can't be all coincidence
and based on the blatantly pornographic nature of the Caveman Omake, I wouldn't be shocked if Mashima is a porn artist in his off time