The most scariest enemy

[Xanatos]

I would say the water lurker (or whatever their official name is). Other monsters were scary too but although they primarily appeared in darkened areas I still found myself entering & exploring them (albeit very cautiously). But once I encountered the water lurkers I steered clear of water throughout the entire game and dreaded any moment I had to get my feet wet.

And, unlike other enemies, there's no hiding from the lurker.