Trivia: Director Guillermo del Toro disowned the theatrical edition after repeated creative clashes with producer Bob Weinstein, who notoriously made numerous and often unreasonable demands of the production, including dictating which scenes del Toro could shoot and forcing del Toro to cut several key sequences in editing, which lead to to many of the intended themes of the film becoming lost. Years later, del Toro was granted permission to use what little deleted footage could be recovered to make a pseudo director's cut that re-instated several key scenes absent in previously released versions. Guillermo del Toro has since said that he is proud of and enjoys this second edition of the film, as it is closer to his intended vision, though it is still missing some sequences that he wasn't allowed to film.
Trivia: In the original ending as intended, the "King" of the Judas Breed wouldn't be killed by the train, but would rather in silhouette form back into a human "mimic." He'd step out of the shadows to reveal the perfect image of a nude human male. He would then point at Susan and in English tell her to "leave", implying that the Judas Breed had reached such a stage in their evolution to be able to perfectly mimic human beings, even down to speech patterns. This was in keeping with the original, much more apocalyptic tone of the story before script-changes forced by the producers.
Trivia: According to director Guillermo del Toro, the instant he knew he was going to have problems making the film was when he submitted a test shot of the monsters, which are giant insects. The studio then contacted him and complained that the giant insects... looked too much like giant insects.
Trivia: An unused alternate ending for the film would have Susan and Chuy escape into a subway platform, where they'd be greeted by dozens of men in long coats, with it being left vague whether or not they were humans or "mimics" of humans. This ending was reportedly even shot and shown at a few test-screenings, where it proved to be unpopular, and thus was replaced by the more "happy" ending in the theatrical and director's cut releases.
Trivia: Originally intended to be a 30-minute short as part of an anthology film, though it became so ambitious it was made into a feature-length film.