Some kind of ghost would be good. I've suggested in another thread that abandoned mines should have undead miners, or living goblins. Since NPC villages are most likely happening, maybe those villages could have cemeteries. At night, depending on how many graves there are, a certain amount of ghosts appear out of the graves and start to wander the area going further and further from the village until day comes when they just disappear. An interesting mechanic would be if a villager dies, a new grave appears and one more ghosts spawns at night from then on. Clever peeps could risk themselves to try finding villages at night this way hehe
I'm not sure how many villages there will be, but I assume you won't find one in every valley... so ghosts will be somewhat rare, and thus should be really difficult to deal with. Requiring some special powder from some dried and ground plant be added to weapons or something like that. Anyways, I am loopy and need sleep lol
I posted one thread that listed a bunch of biome types I thought would be great, and along with that predators and prey for each (not a complete list..) and some monsters for each if I recall correctly. Here it is. Nobody responded to it so I didn't go further with it lol. But you may like it there ArcticuKitsu. Looking at it now, it was more about creating an ecosystem in each biome. Instead of monsters, prey and predators found everywhere etc.. they were in specific biomes along with the flora being different, another reason to explore....
Zombie dog should be the pet of some undead monster IMO. Which maybe, with much effort, we could make it our pet?