At the end of the film, the wolves from Rosaleen's dream apparently enter into the real world, congregating outside her door and waking her up. As she lies in bed, terrified, a wolf crashes through her window, and the film ends with the sounds of Rosaleen screaming.
This unusual ending has prompted more debate amongst fans and critics than perhaps the rest of the film put together. According to the 'Behind-the-Scenes Dossier', the end is a depiction of the final "end of innocence, demonstrated by the wolves that crash through the sleeping girl's window, knocking her toys and her childhood to the floor." We have seen Rosaleen accept adulthood in her dream, and now she is being forced to accept it in real life as well, and it is terrifying to her. As such, a basic interpretation of the scene is that it continues the metaphor established in the dream - the wolves stand for adult sexuality, the difference is that in the cold light of day, that sexuality is more intimidating than it was in an idealised form in the dream.
However, there is more to support this suggestion than merely the fact that in the dream Rosaleen turned into a wolf. Much of the film seems to posit a parallel between female puberty and lycanthropy. For example, when a girl enters puberty, she grows hair in unusual places (just like lycanthropes) and she goes through a monthly cycle (just like the moon; which is also important for lycanthropes), which can often make her bad tempered and aggressive (just like lycanthropes). At the beginning of the film, Alice tells her parents that Rosaleen is in bed complaining of "tummy ache"; could this tummy ache be the cramps of her first menstruation? If this is so, then Rosaleen is presented as having little choice in the matter; adulthood is taking over whether she likes it or not, and this is represented by the violent entry of the wolf into her bedroom. It is not a gentle and consensual acceptance as it was in the dream; it is sudden and terrifying, and Rosaleen is trapped, unable to escape from it (represented by the fact that there are wolves outside her door and wolves coming in through her window; she has nowhere to run). Significantly, as the first wolf bursts through the window, there are numerous slow motion shots of Rosaleen's childhood toys being destroyed, suggesting that her childhood is over and adulthood is about to 'consume' her (i.e. the advancing adulthood represented by the wolf is destroying her childhood, represented by her toys). Just like in the dream, Rosaleen is set to become a wolf; ie, she is set to become a sexual adult. Indeed, this seems to be confirmed by the fact that, if you look very closely when Rosaleen is screaming, you can see that her maxillary canines (her upper canines) have become slightly elongated, thus suggesting that her transformation into a wolf is beginning. Just as we saw in the dream, Rosaleen is becoming an adult.
However, in the dream, this transformation was presented as a positive thing, but in real life it seems to terrify her. Does this suggest that she still fears adulthood despite her acceptance of it in the dream world? In the dream, she is presented as willingly choosing to join the Huntsman, but in the real world, the appearance of the wolves terrify her. Why is this? Interestingly Neil Jordan comments on this very question on his DVD commentary. As the wolf bursts through the window, Jordan says, "Now if we're being entirely logical, she should look at this creature the same way she looked at the huntsman, with that same curiosity." In the dream, Rosaleen is fascinated by the sexuality of the Huntsman, and, by implication, her own developing animal sexuality, but in reality, when presented with something similar, she shirks away from it and resists it. Most fans read this simply as representing the fact that it is one thing to dream or imagine something, but it is entirely different to actually experience that something for real. In the real world, Rosaleen is wearing lipstick and her sister's dress. Clearly she is fascinated with adulthood, and as such, she dreams of her acceptance of the sexual nature of such adulthood. When she wakes up however, she realises that what she was dreaming about is really happening to her (hence the breakdown of the divisions between the dream world and the real world), and whilst in the dream she was given a choice as to whether or not to accept it, in reality, no such choice is possible; it is an onslaught she (and all children) are powerless to resist.
As with the she-wolf story, the violent entry of the wolves into her room, the shattering of her childhood toys, her screams, and her growing canines represent that fact that her childhood is over, and can never be returned to; the 'corruption' of adulthood is absolute. Her childhood innocence (represented by the shattered toys) is sacrificed so as to make way for her adult sexuality and maturity (represented by the wolves). With all of this in mind then, her screams seem simply to be her realisation that the choice she made in the dream is not a choice at all in reality, but is something she cannot avoid, and as such, is scared by it. Whatever the case about her scream however, the general consensus amongst fans of the film is that the entry of the wolf into her room is a continuation of the metaphor established in the dream - wolves represent adulthood and sexual maturity, and Rosaleen is about to begin experiencing both.