In 1956, the company that would become Marvel Comics began publishing a humor magazine called Snafu. Since The Saturday Evening Post proudly claimed that it was "Founded by Benjamin Franklin," writer Stan Lee claimed that his magazine was "Founded by Irving Forbush, Losted by Marvin Forbush." Not a great joke, it nevertheless would remain with comic fans for many years.
Lee ran a column in the original Marvel comics. He called it "Stan's Soapbox" and penned it in the style that permeated Marvel well into the 70s and 80s. The Soapbox featured bombastic but friendly-sounding copy which talked about Marvel’s staff as though they were a social group (many of them, in fact, worked at home), and created a sense that the reader was being invited to join. Irving Forbush was a part of that, an in-joke that fans shared, even if none of them really knew who Forbush was.
Lee and others often mentioned Forbush in the letters column and Bullpen Page, and Marvel’s fans started mentioning him as well. He occasionally appeared in the credits at the start of a comic—- allegedly assisting, for example, with art. These references reinforced the idea that he existed and held some position with the company.
Forbush soon appeared within Marvel's comics, as well. Several celebrity-gawkers may be found among the crowd gathered around the Baxter Building for the wedding of Reed Richards and Sue Storm in Fantastic Four Annual #3. Among these we see the Brooklyn Chapter of the Fantastic Four fan club, fans of Millie the Model, and a partially-identified group desiring to see Irving Forbush.
Marvel’s 1960s parody comic, Not Brand Echh featured Irving and his super hero alter ego, Forbush-Man. This character clearly shows the influence of DC’s original Red Tornado. He wears long red underwear emblazoned with an F and a blue cape, and he masks his face with a pot with eye-holes. In this incarnation, Forbush worked as a custodian at Marvel Comics. He tries to get super hero's autographs, but constantly fails. His costume appears to have been an attempt to fit in with his idols, though a more direct inspiration is his aunt, who hit him in the head with a pot while he was doing dishes.
In the 1980s and 90s, Marvel relaunched the Not Brand Echh concept as What Th--?, and Forbush-Man made occasional appearances. Largely, however, he is an old fannish gag, a remnant of the original 60s era Marvel, a vision of the company which no longer exists.
By now, however, the jig has long been up. Stan Lee discussed Forbush's history and fictional nature in a late-1970s Stan's Soapbox.
"Forbush Man." Marvel Database. http://www.marveldatabase.com/wiki/index.php/Forbush_Man
Stan Lee, Jack Kirby et al. "Bedlam at the Baxter Building." Fantastic Four Annual #3, New York: Marvel Comics, 1965.
Don Markstein. "Not Brand Ecch." Don Markstein’s Toonopedia. http://www.toonopedia.com/echh.htm