If you haven't seen it, there's an amazing video on YouTube about the effects that wolves had on the geography of Yellowstone National Park.
Yes, the geography.
It's worth the watch, but in short, wolves prefer to hunt in areas where they can easily ambush prey. As a result, this keeps deer out of valleys and gorges where they can be easily trapped by natural barriers like cliffs. Without hordes of deer stripping everything down to shorn grass, areas to sprung up into dense thickets.
In some cases, whole valley systems turned into forest in less than a decade.
This allowed a whole forest ecology to spring up - birds, beavers, rabbits, mice, hawks, foxes, badgers, bears, etc.
Since forest roots stabilize soil against erosion more effectively than grass roots do, the rivers tend to straighten, following narrow, deep channels instead of shallow rivers that meander their way into loops and bends as a result of rapid erosion.
The presence of wolves changed the rivers.
Why Not Monsters?If wolves can do this, why wouldn't monsters?
Imagine a pack of lion-sized wyverns. They spend ages on the wing, in great flocks, descending on anything meaty that lets itself get caught out in the open.
With no way to evade them in open grassland, grazers must stay near enough to the edges of forests so they can run for cover. As a result, whatever saplings do manage to sprout further away no longer get stripped of their soft bark and shoots, they manage to grow and develop into copses.
Eventually, you wind up with a patchwork; large copses of tall trees, separated by wide lanes of grasses.
Stripped of their saplings and underbrush, the forests are robbed of their next generation, and eventually turn into skeletal stands of dead trees - which make perfect wyvern roosts.
The grazers are forced into other areas
The grazers strip the copses of everything young, though, so you get these relatively clear, mature forests that die off after a hundred years. In a few decades, these turn into stands of bare, dead trunks. Perfect for wyvern roosts!
Eventually, these stands regrow with underbrush; nearby deer strip other copses of their undergrowth, and so deer and wyvern pack alike move on, beginning the cycle anew.
What's important about this isn't my ability to predict the evolution of a complex ecosystem, maybe I've got it all wrong.
But who cares - we now have an interesting telltale of the presence of wyvern packs: copses separated by lanes of grasslands, with occasional stands of dead trees choked with underbrush.
This is very distinctive, enough that players could soon learn to recognize it as a sign of wyvern activity.
I turn it over to you. What other terrain effects could monsters exert? Whether or not you get as far as the resulting ecology, what are the raw pressures that various creatures could exert?
- Imagine giant blight ants that only eat mature trees (they do something with the wood, involving fungus).
- Imagine river-swimming herbivores that can come just far enough onto land to remove riverside vegetation.
- Beavers cause flooding with their dams, perhaps certain underground dwellers divert underground water sources.
- Foragers that turn up the soil. (Ankheg burrowing brings in flocks of birds, looking for freshly revealed worms.)
- Ambush chasers prefer close, confusing terrain, driving favored prey elsewhere.
- Airborne hunters prefer sparse, open ground.
- Droppers need overhanging branches or cliff faces.
This is of course to say nothing of the weird shenanigans that intelligent monsters can get up to.