Monsters, i.e. fauna that prey on humans, make all sorts of ecological sense. Most forms of life on earth has something else that kills it and eats it, controlling its numbers. Most top predators face Malthusian existences, they grow in numbers until their prey declines in numbers enough to starve them. It is a little more complex than that, as we are learning that we aren't the only top predator that manages its own ecosystem, but while other top predators in the world have total numbers in the thousands or tens of thousands globally, there are eight billion, give or take, of us.
It would make all sorts of sense for there to be an ecology in which vampires, or dragons, or zombies, or hydras, or alien overlords, or angels and demons, or giant spiders, or triffids, or what have you to kill off large number of humans on a regular basis (millions or tens of millions of people a year), or for us to struggle to survive despite the constant threat of predation.
Of course, this isn't what happens. Even before we became modern humans, we finished off much of our mega fauna competition, and by the time our current species became dominant, most of the last few competitors for the top predator spot were wiped out. A Malthusian limitation on global population became obsolete right around the time that Malthus formulated it.
We are no longer a species whose population is limited mostly by disease, starvation and war. We have gradually found ways to cure and prevent disease, live peaceably with each other, and feed ourselves with a small fraction of our available time and treasure. Rather than restraining our population mostly through death, which claims quite a bit less than 2% of us each year, mostly at quite advanced ages, we have restrained our population by restraining our fertility, mostly in some semblance of a voluntary fashion.
Monsters could surface. I'd put my money on microorganisms that cause plagues ahead of mega fauna, but something could kill us (although I'd put my money on modern medicine and epidemiology in the face of threats like those). And, while we seem to have navigated every plausible terrestrial natural disaster, we know from our own planet's history that big chunks of stuff that slam into us from space could mean game over (although we're increasingly at a point where we can defy even that kind of threat).
Selfishness and an affluence meme, rather than monsters or disease or starvation or war is the main means by which we limit our population, and it seems to be an ironclad law of economic development that increased economic affluence through development naturally produces a reduction in fertility rate, often to a level below the replacement rate.
While the world's population is still growing, the growth rate is falling as economic development makes a seemingly inexorable advancement in the less developed parts of the globe, and eventually, if current trends continue, the world population will first stabilize, and then shrink, as we as a species collectively decide that life is nicer in a world where fewer of us share the same finite pool of resources. Perhaps at some point, a natalist meme will overtake the affluence meme that is keeping population growth in check. But, we aren't close to being there at a global level yet.