(belated) trick or treat! 🎃 i'm baby and terrible with horror, but i'll +1 mononoke! it actually starts in another series called ayakashi, japanese classic horror -- i believe it's the last 4 eps? but it's very good, and it likes to dip into surreal horror?

and people have mentioned perfect blue, so i'll drop in paranoia agent, which isn't Scary scary, but it's unnerving enough that it really stuck with me after i watched it!