Please don't be a hater
I don't understand why so many people hate this movie. I'm a die-hard horror movie fan and nothing scares me anymore, but this one was at least suspenseful.
Allow me to put this in perspective... Too many horror movies these days are incredibly lame. For example, the latest Blair Witch movie, The Forest, Lights Out, Zoombies, and American Conjuring (cringe). These movies are filled with tons of jump scares because there isn't anything else in these movies that is truly horrifying; there's just no substance. But, this movie was horrifying simply because the characters were paralyzed with fear and you felt that fear knowing the monster could attack at any minute. With no weapons and no way out, the mother couldn't protect her child, no help was coming, and it seemed inevitable that they were going to die.
So, the screenwriter of The Monster didn't explain where the monster came from. Do we really need to know? Sometimes "less is more." The movie provoked a good bit of emotions, too, because the mother went from thinking that her daughter was simply an inconvenience in her life to later sacrificing herself to save the child. Was this as good as "The Descent" or Ridley Scott's "Alien" movie? No. But it was far better than most I've seen lately. It is what it is: good entertainment. :)