From Tales of the Questor: Rosad Athair Beither, a Rac Cona Daimh scientist 300 years ago, while helping fight off the many dark creatures of the world, came up with a startling theory: that the supernatural creatures of the world all sprung from a single source, the Patronum Monstrum. He spent years proving that all you needed was sufficient knowledge and skill with biomancy and lux craft to create the various horrors of the world, and to do so, he did horrible, horrible things, experimenting on both animals and people, and the things that he created... sometimes escaped. When he was caught, the High Council kept him alive for a year so he could collect his notes and sort them for future study, before he was finally executed for his crimes. The most terrifying thing about all of this story, even more than the lengths that he went to try and prove his theory is simply this: NO ONE HAS EVER SAID HE WAS WRONG. Not a single piece of evidence, in-universe or out, has led us to believe that this theory is wrong. There's even the Fae, a race of nigh-invulnerable, incredibly powerful beings stated to actually have been created in ancient times as a slave race for a long extinct people. If it's possible for even a group of beings as powerful as those to have been created by people possessing sufficient skill and knowledge, then it's not so far a jump to believe that someone, or something, somewhere, is out there pumping out all the horrors of the world, simply because he enjoys causing suffering and misfortune to others.
*cough* That 'single source' would almost certainly be us. Not us as in humans: us as in 'the sentient races of the world'
The author started recently posting pages for what could be called a monster almanac. Read the pages concerning Ghouls (humans forced to eat meat that was partially eaten by an ogre. Ogres often FORCE captured humans to do this), ogres (which are produced when a ghoul feeds on a dead ogre, most often the one who created it, completing the cycle), and giants (children born from mothers who, due to severe famine, ate rotten fruit that was infected by a mold that causes the unborn child to come out... wrong). Now, ask yourself, what kind of sentient being would knowingly make something like that and intentionally unleash it on the world?
The entry on Dragons is especially terrifying: these creatures aren't born, they are formed in a a pit that spontaneously forms in the earth. Serpents crawl into the pit, and over several days, a dragon emerges. These are not naturally existing lifeforms, but a creature that should not exist in the natural world. This is a rare occurrence, but it has been witnessed having happened recently enough to indicate that this still happens regularly. Something out there makes dragons, and makes them regularly, making certain that the dragon population remains steady.
You know how else dragon populations remain steady? If a dragon is slain, every dragon in the surrounding area goes into a frenzy, destroying everything around them. So, it's ultimately considered, by those who understand dragons, easier to just let one dragon harm your realm, than have a dozen angry dragons completely ravage it.
Tied to the above: Remember that lab full of bubbling beakers, dark tomes, and oh yeah - a damn Rat King? Covered in cobwebs as it was, didn't it strike you as funny that so much of it was intact? More like it had only abandoned for a few years or even just a few months? The most recent chapter shows that the Expansionist Party - the guys whose plans Quentyn ruined twice - has ties to Rosad's Disciples. So he actually ruined their plans three times. And now the Junior Questors are taking their shot at them. Also turns the White Stag's blessing of Quentyn into Fridge Brilliance; why inspire a Questor - and as a result a resurgence of Questor imitators - now? Because a powerful conspiracy is almost ready to plunge the world into a Magi Tech war.
An In-Universe example: What the heck would scare a bogeyman? Answer: wights. Rat wights.