Most of the monsters... are based on some sort of mythology. Every culture and even some geographical areas have monsters and mythology that is their own.
My characters surprise me constantly. My characters are like my friends - I can give them advice, but they don't have to take it. If your characters are real, then they surprise you, just like real people.
Perfection is an unattainable goal. It isn't going to be perfect. Just get words down on paper, and when you stumble to what you think is the end of the book, you will have hundreds of pages of words that came out of your head. It may not be perfect, but it looks like a book.
Readers respond to every genre intensely, if it's a genre that appeals to them. Again, who can say why anyone enjoys horror and dark fantasy? If I can't answer the question for myself, I wouldn't dream of trying to answer it for others.
Two things I do well in books are sex and violence, but I don't want gratuitous sex or violence. The sex and violence are only as graphic as need be. And never included unless it furthers the plot or character development.