My players just did a side quest to hunt monsters in a forest, and they said it was some of the most fun they've had at my table. I'm toying with the idea of doing a Witcher-style monster hunting one-shot (or even campaign) after our current campaign is over, but I'd like to subvert expectations a bit.

I've always thought of most monsters as just a different kind of animal that lives in a certain environment, and in a lot of stories I've read the monsters attack because people are just going through the wrong place at the wrong time (aberrations and undead aside, of course). However, a lot of monster-hunter-type media has the heroes plowing through monsters and harvesting their corpses for power and making weapons and armor out of their remains. This mechanic rules, to be clear, but I guess I'm wondering if there's a larger story I can weave into this well-known trope, while also bringing some "humanity" to these creatures instead of just having my PCs mow through them all.

One of my friends suggested that the monsters are captives in a zoo/laboratory and are being experimented on by a mad scientist Dr. Moreau-style, and that's where the aberrations in the area are coming from. The PCs could be hired by the scientist to capture monsters that escaped or to track down new specimens for his experiments, and they'll eventually figure out the messed up things he's doing. I appreciate any ideas or suggestions y'all have, and thanks in advance for the advice!

Maybe you can have it that some powerful primordial being return and its turning nature against civilization, including beasts and monsters. This would be a lot higher stakes but I think it could work out really well.

You could try to tap into the common Monster Hunter story trope of a new, unexplored frontier where there is no established civilization (that anyone knows of obviously) and the creatures have had Jurassic Park: Lost World levels of freedom to just run the show.

Players could directly influence the growth of the frontier by taking out apex predators and such then as this unfolds you can keep an eye on common threads that might get woven into a campaign arc.

Monster Hunter itself gives you the option of capturing rather than killing (and I guess all the body parts you carve off eventually grow back...?). You could do this with the goal of studying the monsters or even helping with their conservation.