I not sure if I understand the cultural manifestation of our nightmare part but I can only see peoples nightmares as being things that they personally fear such as a certain person or something they don't wish to lose. Nightmares in my opinion manifest and grow by the person creativity and what they can imagine. I remember when I was young, kids my age were taking about being scared of the boogie man or the monster underneath bed can be manifestations of nightmares for children.
There are a lot of monsters that catch my attention like werewolves, ghost, and vampires and many more. I guess one of my favorite monsters are werewolves . I find it interesting that they seem to suffer so much pain when they transform but they look so cool with the things can do like have run fast, have an enhanced sense of smell and animal instincts. Another thing that I like is that most werewolves can only transform during a full moon. Even though they are cool to me and also very dangerous because at most times they can never control what they do well they transform.