I think verisimilitude is one of the most important parts of any fantasy setting. Regardless of other elements, no matter how fantastic or mundane, the world should feel like a real place. One of the best ways I’ve found to give a fictional setting this kind of feeling is to incorporate a few fantastical creatures into the otherwise mundane flora and fauna of the overall world.
Too often it seems like many monsters that could and should be woven into the fabric of a setting are instead used as one-off encounters; their existence is never foreshadowed prior to the encounter and they are forgotten about as soon as experience points are divided up. In the paragraphs that follow, I’ll use some familiar, old school Dungeons and Dragons monsters to illustrate what I mean by making a monster indigenous.
In general, I am not talking about highly intelligent and/or solitary type monsters, although such creatures could and should be woven into the local lore if they exist. (Any locals who have lived in an area long enough would likely know that a manticore roamed the high peaks of a nearby mountain range, and they would probably just avoid its territory.) Rather, I want to focus on slightly more mundane types of creatures that, with a little work, could be realistically incorporated into a local ecosystem.
One of the first monsters that comes to mind for me is the displacer beast. Their description states that they are “very rare” and that they “stay far from human habitations.” But what happens when those human habitations start to encroach on a once remote area already inhabited by these creatures? As the local lord pushes his boundaries and human settlements begin to encroach on previously untamed wilderness, hunters and woodsmen would no doubt begin to encounter this creature.
Such a creature would likely be more than a match for a solitary woodsmen, but a skilled hunting party or an exceptionally talented fur trapper could conceivably deal with such a threat. Also, let’s not assume that these beasts would actively hunt humans. In fact, it is likely that they would choose to avoid contact with this new threat. The humans, however, would probably do the opposite, hunting and tracking this new beast, and creating a new market for “Shadow Cat pelts.” Depending on how magical your world is, these pelts could be eagerly sought after by wizards for use in creating displacement spells and items, or they could simply be a trendy garment among elements of the nobility, such is their rarity. The point is that the creature’s existence becomes part of the world around it, and in the scenario given, has a tangible impact on both the economy and local professions.
You could take this a step further even. Maybe displacer beasts aren’t “Lawful Evil” in your world and are instead “Neutral” like other wild cats (this is the case in my world). Maybe the locals have became so adept at hunting this creature that it is endangered, and a nearby order of druids feels obligated to step in and protect the poor displacer beasts from extinction. Again, the key here is to make the creature exist outside of the vacuum of a solitary encounter. Make it feel like it belongs in your world.
Another great example that I have used is the lowly carrion crawler. This guy is just begging to be made an indigenous part of the world. Mindless scavengers, they exist only to find and consume carrion, and actually provide the same benefit to the ecosystem that other, more mundane scavengers do. To the well armed and well prepared, the carrion crawler is a nuisance to be sure, but to the ill equipped and unwary they can be deadly.
In my world, I changed the climate/terrain of this beastie from subterranean to temperate, making it a burrower that inhabits the forests and plains of a large populated region. Inhabitants of this region call these beasts “Crawlers,” and know what they are, what they eat, and how best to avoid them. While indigenous to the region, they are still a somewhat rare sight, but at the aftermath of a large land battle where the dead were left to rot on the field, it would not be uncommon to see a dozen or more of these monsters crawling along, picking their way through the corpses.
It may seem like a simple, or even silly thing, but one of the more gratifying moments I had while running a campaign in this world came when, after the party’s camp was ambushed by goblins in the night, one of my PC’s said “We either need to move our camp or do something with these bodies, unless we want to be dealing with Crawlers come morning.”