It's a shame books can't currently incorporate images as it would be nice to see stories accompanied with illustrations ingame. I like creeping slime although it feels more like some nameless lovecraftian abomination than a slime

Why go for "friendlier" versions? I figure it can't do any harm to come up with something horrific enough to give nightmares... if you can even get to sleep

Unless stories go along the lines of "The White Mage got herself dissolved when she hugged a slime that she thought looked cute and adorable but later discovered was also highly corrosive"

Well now that I think about it there are several ways mobs can be made more terrifying. For example is it possible to have a different mob animation for when it's aggro'd and when it's not. This means that potentially you can have things like a statue that can come to life, disguised characters showing their true colours or even a parasite exploding host and generating some horrific monstrosity.

I'll also add that if you want to make things truly scary you need the right sounds. For example most mobs when aggro'd at most make a kind of flat note at best. How about some shriek or foul cry to generate terror? Can they also do footsteps?

Then you can add something like right background music. Something like could work although admittedly you need to make sure what's used doesn't breach copyrights etc

Then you just need to design a map where it feels like you're taking a step into inknown with perils awaiting you where every step is potentially encountering new danger (maybe add linked mobs with really long detect ranges for ambushes, with statue example you can walk past statues and provoke a mob at end that gets linked statues to also attack).

Maybe a touch offtopic as we're showcasing artemis' excellent artistic talents in this thread but I figured I could at least describe what I meant

As far as aggro animation isn't it simpler to have 6 animation states:
Normal idle
Normal move
Normal attack
WTF?! idle
WTF?! move
WTF?! attack

If there's no transformation then WTF?! and Normal are identical. If you want a mob that transforms after a certain amount of damage is taken etc it changes to WTF?! animations. Presumably you could even differentiate between a player aggro and a NPC aggro so you could have two NPCs moving and sparring like "normal" humans and then player goes nearby and they change to a "WTF?!" creature and attack. Having said that I guess it's possible to do in lua where you simply remove previous mob and spawn a new one on it's previous location.

As far as sound goes I guess we can look forward to it in 2 weeks time

It also depends on programming limitations. For example with an object oriented language you can have an overloaded methods and inheritance that can easily recycle a lot of code. You essentially have 6 states but root behaviour is all based on 3 basic states but 6 aggro animations. Fundamentally WTF?! idle and normal idle would use pretty much same AI/implementation with at most a few slight modifications.

As far as permanance goes it depends on case. For example a werewolf style mob could perhaps freely revert between states. On the other hand if you imagine something like an alien parasite bursting through an NPC's stomach that's rather irreversible. It could even theoretically extend to invisible mobs that need to lift their invisibility to attack (like a klingon cloaking device ). If you want to go further you can even say that in some scenarios WTF?! state can confer additional effects such as invincibility or even weaknesses but also may have a limited duration meaning that attacks need to be timed for when mob is temporarily weakened.

As an added thought if there ever was a berserk spell that could theoretically transform a mob into it's WTF?! state and make it attack a random target in which case it applies for spell's duration. Having said that it currently seems like quite a bit of work (especially if all mob animations need updated or WTF?! versions) so probably something for 2 weeks time unless a mapper wants to make them specifically for their maps

Nice job, that looks cool.  I like the first one the best, with the dark back ground.