WHEN MIKE MIGNOLA sketched a young demon back in the early ’90s, he had no idea he was about to make a major contribution to comic-book culture — or that the demon himself would be celebrated upon reaching its 20th anniversary.

The comic-book revolution of that decade, though, as stoked by Image Comics, had changed everything. Creator-owned content was hot. So Mignola joined a group of artists who were doing creator-owned projects for Dark Horse Comics (a group that included Frank Miller, Art Adams and John Byrne) and introduced the world to Hellboy.

“I’d been doing comics for 10 years, so I thought this would be a great chance” to introduce Hellboy, Mignola tells The Post’s Comic Riffs. “The environment seemed right.

“I figured I would do this four-issue miniseries, and if it worked, that would be great,” Mignola continues. “Because I did create a book that I’d be very happy to continue doing, but I figured the odds were against me. So I was mentally prepared to go back to DC Comics, and draw Batman books or continue doing something else at Dark Horse, but I figured it was my one shot at creating my own thing.”

Today, Hellboy remains more than relevant in mainstream comics. He’s had two major feature-film adaptations, and Mignola says there’s no end in sight to the comics — despite his “killing” Hellboy and sending him to hell.

Saturday is being promoted as “Hellboy Day,” an occasion intended to celebrate Hellboy’s 20 years. Upon Hellboy’s debut, Mignola said that being associated with some of the bigger-name talent putting out creator-owned books at Dark Horse was a major asset to his title.

“I benefitted, and Hellboy benefitted, from being in this group, and being associated with these guys,” Mignola tells ‘Riffs. “It was very much a case of being in the exact right place at the exact right time. And I don’t know if any of that would have happened had the Image stuff not made the creator-owned thing such a viable option.”

Being aligned with comic-book stars has advantages, but Hollywood certainly has its own. When Hellboy first hit the big screen a decade ago — “Hellboy” was released in 2004 and “Hellboy II: The Golden Army” four years later, both from director Guillermo del Toro — a comic-book movie wasn’t quite as likely to be a blockbuster as today. Mignola acknowledges that the Hellboy movies were a major accomplishment — and were no easy feat to get made.

“Not only were comic-book movies not as common as they are now, but Hellboy is such a hard sell as far as a comic-book movie,” Mignola tells us. “It’s not a mainstream film and it’s not a superhero film. It’s a very weird thing with ‘hell’ in the title, which turned out to be a bigger stumbling block then I ever would have imagined. I never really thought when Dark Horse optioned it from me that it would ever be made.”

“I’d love to think I’d still be doing this comic if there hadn’t been a film, but who knows,” Mignola continues. “The film, on a personal level, it was just exciting to be able to work with Guillermo del Toro, but let’s be honest, it’s very good for book sales, and it’s been wonderful for making this odd character with an odd name much more mainstream than it would have been if it would have just continued with this not-terrific-selling comic.”

But the movies, of course, overcame potential studio and audience obstacles.

“To me, the name Hellboy is so silly and it’s so funny — I would have thought any nervousness people had about the word ‘hell’ would be completely defused by the fact that he’s a big, lunky guy called Hellboy,” Mignola says. “It’s silly — which is why I like the name.”

As for Hellboy’s future, Mignola says he’s not sure when he can let the big red guy relax.

“Well…he’s in hell. It’s interesting when you have a character that you’ve already killed off,” Mignola says. “I thought I was going to simplify his life when I got him into hell, but it turned out there’s a bigger story there than I thought there was.

“Right now, I’ve probably plotted four trade paperbacks — 20 issues worth of material,” he continues. “That’s my focus . Eventually I’d like Hellboy to find a nice corner of hell with a well-stocked bar and a big library and a comfortable bed and let him peacefully retire — but I don’t know.”