A few more ideas, including some intelligent races. These should make evil warlords more and less happy with their armies, especially if they are a different race than their soldiers.

Kobolds per 5 square miles

500 Mammals, including most of the player character races, entering the area gain small scales or start whining like dogs depending on which kind of kobold. The scales are useless as armor and in fact itch, reducing Dexterity bonuses by 1. The yipping can be controlled with concentration, but if allowed to continue, they draw more wandering monsters.

1,000 Altars of kobold gods appear in their central chambers. All kobolds who know they exist and are within the same room gain a bonus to their attacks and movement rate.

5000 People attempting to hide from the kobolds must save (versus death, fortitude or constitution) or be transformed into kobolds, losing the memories of their old life and joining the tribe fully.

10,000+ Traps built by the kobolds in the area do double damage or have a –4 to the saves to resist them. Lesser dragons or canine monsters (yeth hounds are common) spontaneously generate once per month and go out to do the kobold god’s bidding.

Hobgoblins per square mile

100 Shadows deepen and attempts to hide in them improve. At least that is what most thinks as the eyes of hobgoblins easily pierce these shadows.

500 Most objects clank as if they were made of metal, even if they are just being moved and not striking one another. This increases the chances of wandering monsters but also makes determining who or what is moving by listening impossible.

1,000 All bladed weapons gain an even more keen edge, doing one more point per die of damage. They also drip dark blood and this makes tracking easy. This may seem like a downside for the goblinoids, but they are in such numbers that they leave other signs as well, so they care not.

10,000+ All writing in the area turns to goblin script permanently and additional sentences praise the goblinoid gods. Hobgoblin shamans can cooperatively summon warbeasts such as trolls, dire carnivorous apes and dire hell hounds. The rituals are quick but also drain the shamans’ other spells. The ground turns to mud that impedes the movement of all non-goblinoids. And finally when a hobgoblin strikes in battle, its weapon pulses with light and potentially blinds the target.

Shambling Mounds per 10 square miles

10 The water sours and produces an odor that makes people ill.

50 Potions and magical food sour and consuming them is difficult. Dead trees are much more likely to fall when something walks nearby, crushing the interloper.

100+ The mud swallows everything and only those creatures that can walk on water, large plants and shambling mounds can move (or stand) freely. Buildings drown and fill with mud, either sinking or becoming mounds.

Lizardmen/folk/people per 100 square miles

50 All reptiles gain the ability to change their color to match the surrounding vegetation.

250 Every person and creature can breathe water, allowing the lizard people to tame unusual land creatures for use underwater. The lizard people gain the ability to see through mud and muddy water as if it was a bright day.

500 Mammals become rare as they give birth to reptile or bird versions of their species. These breed true within and outside the area of effect. The lizard people gain more diversity in their appearance, taking on some of the traits of the new species of birds and reptiles. Some can even fly.

1000+ Dinosaurs start to spontaneously generate. These breed true inside and out of the area of effect. The lizard people gain the ability to shapechange into the medium and large dinosaurs species that have appeared in their lands.

Ogres per 10 square miles

100 Everyone and thing becomes hungry and starts looking at each other as a potential source of food. If an encounter results in the worst category, when combat ends, the feast begins. Even the player characters will eat things they would never do otherwise.

500+ All worked goods degrade. Cavaliers and paladins become embarrassed by the crud they have to wear and wield. People attempting to craft must make an encounter check and if that results in combat, they act much like an ape and break their tools and raw materials. Ogres hate this as all their shiny stuff loses its luster, making them madder and more likely to look for more loot to steal.

Gnomes per square mile

500 Signs and other writing gain a flourish, becoming more vibrant and eye drawing.

1,000 All the pretty colors and amazing sounds start to overwhelm the senses. Surprise is more likely against everyone and creature in the area of effect. Animals that stick around get use to this and rarely react poorly to being surprised, though the same does not apply to curmudgeons. Small items worth no more than 5 cp appear in one’s pockets or other containers, once per person per day. Gnome tinkers who sell out of the area take this stuff off of people’s hands to boost their inventory.

50,000+ Illusions become solid if the caster wishes but their duration is either 1/10^th normal or 1 day, which ever is less. Illusions that are already solid, such as simulacra, are not affected. Dreams manifest in the real world, becoming Shadow based spells or effects. Nightmares (i.e. bad dreams), night hags and the like are banished.

Anthropomorphic Felines per 10 square miles

10 Everyone feels relaxed and all non-urgent actions take three times as long.

500 The need for sleep increases by two hours. This does impact the amount of time a spell caster needs before memorization/preparation.

1,000+ Small dusty bunnies of hair appear in anything and everything. Eating takes twice (actually six) times as long if one wants to get the hair out first. Electrical spells are enhanced by one caster level.

Crabmen per 50 square miles

50 The abundant of crabs, normal sized and giant, increases. Land crabs will start to invade the terrestrial parts of the area of effect.

500+ People start to speak crabman instead of their racial language as well as the ability to breathe water at will. The shells of harvested crabs become more plastic and can be melted and molded into a variety of tools and other implements. Some animals (and I do mean animals, not just mammals) give birth to arthropod variants of their own species. These breed true inside and out of the area of effect. This process does not replace the parent species, unlike the effect of lizardmen, as most births are normal. Sahuagen and ix ray eggs always hatch into crabmen, usually resulting in a swift reduction in the crabman population.

Anthropomorphic Sparrow per square mile (these guys are sparrow sized)

50 Seeds, fruit, insects and small branches become more abundant. Tiny critters have an easier time avoiding predators while in the tree crowns.

250 Branches actively defend birds of all kinds, Entangling all who attempt to harm them.

1000+ Forbs, grasses and weeds start to hover and fly, providing roosts and nests in midair.

Anthropomorphic Sauropods per 100 square miles (thanks to Tim Snider for the idea, these guys are human sized)

200 Vegetation devolves to species of a much earlier era. The local climate becomes warmer.

500+ Small dinosaurs spontaneously generate. These breed true inside and out of the area of effect. Given enough time, they replace the existing small fauna. Mammals have difficulty thinking complex thoughts; resulting in certain skills (DM’s call) have a penalty.