Consider the idea of spontaneous genesis of life, as used to be belived. What strange kinds of life might arise from modern substances? What kind of vermin would a landfill produce, or an oil spill on the ocean?
A demon who isn't some hideous creature bent on consuming souls, bu is, in fact, a likable chap who enjoys having pleasant chats with people - trying to talk them into becoming his servants.
A swamp witch that, when she dies, a huge black worm burst from her corpse and tries to burrow into the nearest living creature to use as a new host. If the the worm is injured in-between hosts smaller black worms swarm out of any wounds and burrow into anything living.