while there are KR monster types that can't necessarilly be described as frightening visually, I don't exactly think that a monster/inhumanoid has to look scary to be scary, since a lot of monsters are frightening in what they do.
Worms come off as scary because with their ability to disguise themselves as humans, it would make people wary of each other i.e. if someone you really trusted or cared for turned out to be a Worm, you might begin to be fearful of just about anybody. While this ability is shared with Fangires, Worms copy actual people, rather than simply hide there true form by appearing human
There are also monsters that can be scary because of how easily and somewhat unstoppably kill people. the two prime suspects here are Orphenochs and Fangires. Orphenochs just extend their fingers to stab a person through the heart, which usually results in death rather than the intention of creating new Orphenochs. Fangires OTOH are frightening because they can kill people without even touching them. The way they kill with disembodiesd fangs is horrible because it's nearly impossible to prevent, since the death happens very quickly and even if a person dodges them, the fangs can be redirected.
Then there are odd examples like Imagin. Making a contract with one is like making a deal with the devil. sure they'll do something for you, but the consequences can be grave. By going into the past, an Imagin can potentially cut short a person's life or alternatively ruin that person's life, and the victim wouldn't realize it until it's too late. and the idea that their appearances are twisted from people's own imaginations is kinda scary to think about.
but of course there are also simpler ones like Makamo who are basically giant mutant killer animals, and I mean mutant is regards to their appearance, not the physiology, and the Undead, who are essentially just monsters created to fight each other for supremacy of the earth. Dopants are a greatly exxagerated form of saying drugs are bad, and lastly Mirror Monsters are just monsters that feed off each other to survive