Fantasy worlds have too many hunters and not enough hunted to be stable ecologies. There are simply too many predators, especially keystone predators. Especially in ecologies that use Wilderness Random Encounter Tables, the hunters would wipe out their prey and starve, according to any ecological analysis based on the mundane and familiar. However, fantasy ecologies are not out of balance. The basis of predator-prey dynamics in fantasy is that intelligent species make excellent prey.
Hunting intelligent species is dangerous, yet fantasy predators prefer intelligent prey. Fantasy is filled with nightmare creatures that only hunt human, or human-like, prey. Such predators not only survive, but thrive, because their prey is more nourishing than natural law allows. For example, Smaug the Dragon grew to hugh size and lived for hundreds of years after a single rampage, in which he ate several thousand Dwarves and Humans. No natural physiology could explain this outcome, yet it is typical for fantasy tales.
Human overpopulation contributes to this abundance of predators. When food and disease are not limitations, Human population doubles every 25 years. However, predation could take the place of the above limitations. This keeps the Human population stable, while supporting a large predator population. Contention between predators becomes an issue only for territory and during periods of scarcity. And scarce Humans seems an unusual event.
Usually, predators on Humans are lone hunters and keystone predators, but there are exceptions. Some predators hunt in pairs or packs, and some are themselves hunted by even larger and more frightening beasts. While it is difficult to imagine a Dragon or a Vampire being eaten by some more terrifying predator, it is easy to imagine a flock of Harpies eaten by a Sphynx. Fantasy allows a food chain of greater height than Human history has allowed, with little possibility that Humans could hunt all keystone predators to extinction.
What, specifically, makes Humans good prey? Is it something tangible, such as the blood, brain, or liver? Is it something intangible, such as the soul, Chi, or an ancient curse by a scheming deity? Is it just Humans, or is it any speaking race, or any race that realizes "I think, therefore I am?" Whatever the reason, the presumption is that the fantastic food chain of fantasy tales is rooted on Humans themselves, supporting predators that must hunt Humans or go hungry.
"Fantasy is what we make of it"...suddenly has a new meaning.