Honestly I'd suggest googling some monster designs and what not or myths from various cultures. I recall googling once a monster name from a video game from Japan and it turns out it's a monster from myth that's this woman that will be in a lake or body of water and act sweetly to lure people to her (not like a mermaid who seduces, since this creature preys on kids and animals) and will drag them down to murder them. A trademark of this horrifying creature is it lets out a shrill scream of sorts when it fails.
Another idea is will-o-wisp. Which are a totally real thing. (Look up documentaries about where the origin of them came from. You'll never go into a swamp or bog, ever, EVER, again.)
Japan has a lot of weird monsters (look at Pokemon for examples), but there's other cultures that have strange things too. Just depends on which sort of vein you want to go down. Since you have fairies of a sort, you could add in dwarves or death ghost things. Wow my brain just did a turd. I think I mean something like a fairy like Peter Pan, but as a death guide...?
Roman and Greek have terrifying monsters too - minotaur, satyrs, and so forth.
I've seen a lot of slime boys and girls before on tumblr from artists and stuff. I also saw a manga the other day that was animal monsters thingies falling in love - so they'd look mostly like people but have several traits that made them animals, like the main character had a large tail that had heart patterns on it.
Disney used creatures called "shadow walkers" in an episode of Aladdin as part of their tv series for it - they look very creepy, and usually are some form of lost person who strayed out at night and became "lost", only to return 3 years later as a monsterious creature with mostly no memories.