All that you dream of, even nightmares, are from your own imagination. It's things you've seen mixed together into something unreal. So-called "nightmares" are usually inconsistent, they don't make sense, but you try to escape something. At least in my experience, I've never seen any monster or whatever it is you want to see in a dream, or nightmare, and even if I did, I could not remember what it looked like when I woke up.
If you want inspiration for art, for instance, you might want to look at other peoples art, watch movies, go to galleries (real, online or both).. heck even read books, if it's in you, and a book describes something scary, you'll generate a picture of that monster and it is just a mater of patience to get it down on paper. A lot of people also find help in staying up late, drawing these things after midnight. There's numerous theories about it, but most artists find themselves to be more creative at night. Personally I feel home at night, the darkness is safe and all the sounds that you don't usually notice, because there's too much noise, come to life. Some say it's the moon too.
One thing I'm quite sure of though, is that trying to find inspiration in nightmares is nigh impossible.
