I've never really heard of many people who could see nightmares. There are two forms of them; the "demon" kind from way back in the medieval times and the skeletal, sometimes winged horses of the dead.
My experience began with them a little over a year and a half ago in December. My mom was in the hospital about to have major back surgery done and I was talking to her on the phone. I screamed because I saw a huge dead looking winged black horse walking down my driveway and it stopped to turn its massive head and look at me. The next day my mom's back surgery went wrong and it left her paralyzed from the waste down.
The next time I saw a nightmare I summoned it. I won't write the summon I used for the fact that I do not believe it's safe to mess with these creatures unless you know how. I only saw a glimpse of it before I went to bed that night. The next morning I found out my dog laying dead on the side of the road; a car hit her the previous night.
So my experience with these creatures has been a little shaky however I believe that it is an amazing experience.