I’ve found that “nightmare” is a classification of dreams that has more to do with the emotions it causes me than it does the actual content of the dream.
A dream about being chased and murdered by demons might not be a nightmare, if I experience it as an interesting adventure or story. An innocuous dream about sharpening a pencil might wake me up in sheer terror and be the worst nightmare I have for a month.
My most common “nightmares” that wake me up screaming or crying are about my family and friends. They often have strong negative energies about them that are sometimes tied to the plot of the dream, but not always. Most of my monster dreams are neutral, often enjoyable, but rarely true nightmares.
If I’ve played video games recently, I tend to have a lot of dreams that would be terrible to actually experience (i.e. fighting monsters), but I experience them with sort of a detached, mild interest. They feel kinda like video games themselves- I know how I want one thing or another to go, and I have a bit of investment in other characters not dying, but that’s it.