I threw out a theory not long ago that suggest that perhaps the nemesis can take the form of dead humans and that jacob can take the form of dead animals. In the first season when u heard the smoke monster the producers said that the roaring noise that you heard was similar to what they used in jurrassic park to make the t-rex roar. They said they used multiple noises made by many different animals to form one loud roar. In some cases with an appearence of the smoke monster you would hear this noice and in others you hear the whispers. I think when the whispers are being heard that it is the nemesis and when the roaring is being heard it is jacob. I also find it hard to believe that a dog survived a plane crash while in the cargo area of a plane (i know it's a television show but ya know it's lost) It;s my theory that it's possible that jacob took the form of vincent the dog to help guide walt who was very special to the island. It also fits because vincent was always running away from walt around the same times the monster was being sighted by other survivors. This would also explain the hurley bird which I also think is jacob and could explain how he can come and go from the island....by flight. I know this is kinda out there but I dont think it's outside the realm of possibility. Let me know what ya'll think.