I just want to point you to Wonderbrary, a very nice site that lists many of the self-identified horror fiction series for children, for those who have powered through the 90+ Goosebumps books. I’m sometimes asked for this information, and this site has done several posts with the express purposed of identifying these series. Unfortunately it does not appear to have been updated recently, but if you’re looking for information on scary series books published prior to 2011, this provides a nice list, including cover art.