I don't write horror but in my own research for an agent of my genre what I've found helpful is to check the Acknowledgements in books in your genre.  So maybe check to see Stephen King's agent...which is Ralph M. Vicinanza, Ltd. Literary Agency.  I only know this b/c one of their agents is going to be at a conference that I'm attending next month...she is available for pitching meetings.  I'll find out who I am pitching to in a few weeks...exciting!

Anyway, check other horror/thriller books and many times you can find info in their Acknowledgements.

AND

Here is a website:  http://www.wrhammons.com/literary-agents-books.htm

On this site you can look for the book or the author and their literary agent should be listed there.

Good luck!