It's not just that it was so bad it was never released—it was never meant to be released. They didn't tell the actors that, though. Here's the trailer (which only played once, before a film called Carnosaur.) The full movie is below.
If you're a fan of Arrested Development* or moviemaking history, you may be familiar with the sad, strange tale of 1994's unreleased Fantastic Four film.
See, when studios acquire the rights to certain properties, like comic books, they come with a "use 'em or lose 'em" clause to keep studios from just hoarding story rights. If they don't make something with it, they eventually lose the license. Nowadays, they just reboot the whole thing every decade. Back in the day, however, they would make films for the explicit purpose of saying, "See? We're using it. Now give us another 20 years on the license."
But the cast and crew of this film were never told that was the film's intent, and they and many comic nerds who had gotten wind of it were understandably excited (and then disappointed) about its release. In 2005, Stan Lee confirmed that the film was, indeed, a copyright trap (although other sources say the studio was paid not to release it in order to protect the brand).
Anyway, as you've probably heard, the most recent Fantastic Four has been a gigantic flop, both critically and financially. The 2005 version did fine financially, but I was there in theaters for it, and let me tell you: it was not good. It was bad. It was never, however, this bad:
*On Arrested Development, Maria Bamford plays the actress who played Sue Storm in this very 1994 Fantastic Four, and she factors largely in Tobias Funke's doomed attempt to create a musical based on the film.