The mbwa mwitu that stopped Emma from leaving Storybrooke has not been aliyopewa an identity, but we have been aliyopewa clues enough to determine it. In a sneak peek for tonight's episode,Graham is often featured with one mbwa mwitu as his companion. In another sneek peak,the Evil Queen later reveals that he was raised kwa wolves. Since everyone in the fairy tale land was affected kwa the curse,even animals, I believe that the mbwa mwitu that kept Emma from leaving is the same mbwa mwitu that was Graham's ally and that possibly raised him.
