Other than the fact that it appears to be a naked, unwelcomed, long-haired female with an enlarged mouth, sharp teeth, a long tongue, and no eyes?
That said, I do feel like there's no difference between the tag horror_(theme) and pool:Nightmare_Fuel, and the two should probably be merged as the latter is mostly just a pet pool. However, I'm not concerned either way.
Just realized it but why is this in Nightmare Fuel?
If anything this is just arousing not scary.
Well, keep in mind Sadoko is just like this (except usually not portrayed as a titty monster). I mean, if you break it down, what's really scary about a skinny girl whose hair is blocking her eyesight slowly walking towards you? The cinematography really has to do a lot of heavy lifting making that seem scary; she's just a step short of trying to portray the bunny from Monty Python and the Holy Grail as an actual horror monster.