The hostile counterpart to Animals: no random spawns, but nests that swarm.
The vision
Random spawning is replaced by a logical distribution, following Biome and Season and anchoring each monster to something nearby: the spider and its web, the zombie and a neighboring structure, the creeper and the forests.
At the module's core, monsters no longer materialize at all. Only nests appear, and each nest slowly seeds its monsters around it. The longer it lasts, the more it produces; destroying the nest dries up the source.
A nest is visually built from pieces of the entity it shelters and of its loot, which makes it identifiable at a glance, with no notification and no marker.
Its age can be read on it too. Visible phases, young, mature, swarming, telegraph its maturity, which allows choosing which one to clear first instead of treating them all alike.
Each nest shelters a matriarch, a reinforced version of the monster that grows stronger with age. She is not hand-drawn: she is generated procedurally from the base entity, attributes, scale, sound, textures, even skeleton deformation, which makes her available for any monster, vanilla or modded.
Her presentation matters as much as her stats: a long, intense cry, a slight camera shake, slowed movement, heavy steps. You know she is there before you see her.
What she leaves behind on death depends on her age and species, which makes the reward proportional to the risk taken, and gives material worth forging.
A chunk-level territory map decides which monster populates which area, and adjacent nests seed each other. Two neighboring territories of different species can come into conflict and weaken each other: it becomes possible to exploit a rivalry instead of fighting everything.
An old nest marks the ground around it, webs, scorch, brambles. Killing the source is not enough: what it left behind still has to be cleared. Eradication becomes a two-part act, and the landscape tells the story of the infestation long afterwards.