Horror is generally not my cup of tea but I got to read an excellent title years ago, "Franken Fran". Surprisingly, no one knows it around me, so it'd be great to have your review if you start reading it ^^! Oh and don't pay too much attention to the covers,the content is actually very different. You'll know what I mean : D
Some would argue it's not exactly a typical "horror" genre, but still, I would also recommend the manga Battle Royale.
If anything interesting comes to mind I'll let you know.
Franken fran is amazing. I haven't read the whole series since I'm too poor to buy it all and don't really have a manga library anywhere near me. But my local book store is pretty good.
Kyuuketsuki Hime Miyu or Vampire Princess Miyu manga was fantastic. Not blood curdling but deep character development and great historical references in a ghost hunter type series.
Berzerk manga is considered action but some of his foes and settings are truly horrifying, especially the second half.
Nano is an ongoing webtoon you can read for free from Naver or line.