The reason why we don't learn what the creatures are, is because they were revealed in the beginning scenes of the movie. They are Sarah's PERSONAL DEMONS. After going through such a traumatic thing, like her husband and daughter getting killed right in front of her, she becomes crazy at the very beginning... as shown in the hospital scene. The lights going out throughout the hall represent the demons coming to get her, and right before they do, she gets away in time.
As they descend into the cave later on, her personal demons come back to haunt her in the form of the creatures. So really, the title ''The Descent'' has NOTHING to do with six girls ''descending down into a cave''.... it actually means that the movie is about Sarah's Descent into Madness. As the girls get killed off one by one, Sarah is the only one that ends up kicking ass in the end, and she is the STRONGEST one out of all of them. The reason why she is the strongest, is because she is the only one that went through a traumatic experience... therefore, she can fight the creatures easier. She ends up going into ''all-out-crazy'' mode, as seen in the climax.
Now, the ending doesn't suck as much as you all think. There was ALOT of thought put into it, and it wasn't some cliche pointless ending like you assume.... You have to seriously examine it, and I know that's a difficult task for most people.
The European ending is better, because it shows that Sarah is STILL in the cave, and that she will never get out. In reality, this is actually a HAPPY ENDING, because she has officially overcome her personal demons, and she ends up seeing her daughter down there with her. If one were observant, they would see that the birthday cake shown throughout the movie with her daughter has only five candles on it, indicating that she was five when she died. However, in the end, there are SIX candles, which shows that this event is a year later, and that her daughter actually LIVED to see her sixth birthday down there in the cave.... in SARAH'S MIND. Sarah never sees visions of her husband because he was a lying cheating bastard and got what he deserved. Her daughter, however, was innocent.
This whole debate about ''Oh, it's stupid how Sarah gets out of the cave and drives the car, and then sees Juno's ghost, and then she wakes up in the cave again, and it blows!!11! LOLZ'' is actually the smartest thing about the movie, because it shows that escaping the cave isn't REALLY what will bring happiness to Sarah. Happiness to Sarah is being with her daughter, no matter where she is.... And now that she has faced her personal demons (the creatures), has overcome them, and was stronger than anyone else... she can die peacefully with the fact that her daughter is down there with her, which means that it's a happy ending.
Now, one could argue that the creatures don't actually exist, and that Sarah killed everyone, since she is the FIRST one to see a creature. However, I hate that idea. The creatures are supposed to represent her personal demons, and the cave is supposed to represent her descent into madness. It wouldn't make sense for Sarah to be a psychotic killer....so TRUST ME.... the creatures were real. They were just a METAPHOR for something else. If you want to get literal... then they were ''Bat People.''
