Only one creature was made for this film.
"The Boogens" isn't an actual term once used by miners to describe the fear some would experience while spending too much time in deep mines, as mentioned in the film. In fact, it was a word fabricated by screenwriter David O'Malley, using the word "Boogeyman" as its root, his original title for the movie but it had already been used.
Rebecca Balding met her future husband, director James L. Conway, while acting in this movie. Conway said in a 2021 interview that he was so taken by her after she auditioned, he told the producer "I could marry that girl." She must have felt the same way, because first week of filming, she proposed to him and four weeks later they married while still filming. They were together until she passed away from cancer in July 2022.
The climax was filmed in a real mine.
To create the roar of the monster, Sound Supervisor Jeffrey L. Sandler asked his son Robert Sandler to come up with something scary. The monster's roar was a dog and cat fight, played backward, and slowed down. To test its effectiveness, speakers were set up at the ends of 4 long hallways at Glen Glenn Sound in Burbank,Ca. With the lights out, Jeffrey and Robert played the effect, and they were both too scared to walk down the hall alone to turn the lights back on.