This one is strictly for those of you who have already seen SHORTCUT, or those who don’t mind spoilers, as the entire post is gonna be nothing *but* one big SPOILER WARNING. Got it? Ite. Don’t say I didn’t warn ya.
So my theory is that the creature, the “Nocturnal Wanderer” (or as it is listed in the credits “Nocturnal Wonderer”—one has to wonder if this was intentional or a mistake made in translation—remember that, although the language spoken in the film is British, this is an Italian movie), was actually the sister of the boy at the beginning, the one who grew up into the dude who started living down in the tunnels, obsessed with the creature, the one who named it and was eventually killed by it. You never saw what happened to the sister. Also, the kids guess that the creature is some sort of “parasite”, right? What if it were some kind of infection that, slowly, over time, altered its host, mutated it? The Nocturnal Wanderer had long hair, too, remember. And as we all learned, there are more than one of the creatures down in those tunnels.
That’s my theory, anyway. Just some fun guesswork. I like that the movie never tells us exactly what the creature is or where it came from, leaving that up to the audience. Nor does it really matter. It’s like, here this thing is, here’s what it does, now deal with it. No exposition and no backstory. None needed.