Uhhyhuh

go frame by frame like with a lot of frames A LOT

i agree with the eyes and teeth, etc comment, but id like to add onto this that things like uncanny valley might help? (unsure how to describe it) like adding something slightly off. it also depends on what type of horror you're trying to depict take this with a grain of salt though since ive never drawn horror myself, though i do really like horror movies/stories you might actually have to do research yourself (Maybe a few searches as to what makes something horrific might help) i dont have a lot of advice here