The japanese "ningen" (sea creatures, not the humans, which are also called ningen).

I have heard about that one. Many mythical water-creatures are spooky as hell imo

I find sirens really unsettling. not because they are scary, but what they can do and the damage they can cause.

I'm not so knowledgeable about sirens. All I know is that they can hypnotize people by singing, if I remember that correctly

Nuckalevees or Wendigos. They’re p much unstoppable unless you have a hyper specific weakness to exploit.

Wendigos and the Abominable Snowman

The Strix from Roman mythology. Basically an owl stretched into the shape of a person. The concept of man-birds (like tengu, or the Toriningen from Yume Nikki) always freaked me out for whatever reason, and that's probably why I find them so unsettling.