In the Monster Manual, almost every monster has a predetermined environment that they live in. This makes it very easy to build encounters based on location. However, I like verisimilitude (the feeling of reality, not realism itself) in my D&D game, and Dungeon Masterpiece on YouTube make an interesting comment in his Geopolitics of Faerun video, which I skip to here:
This is a great question. Are there any Migratory Monsters in D&D?
The easy answer is “Sure! You’re the DM, you can do whatever you want,” but that’s also the boring answer. Of course I CAN do whatever, but I want to know what makes SENSE.
So first, let’s look at the basics of migration. From my understanding, there are three basic types of migration, animal, refugee, and nomadic.
Animal migration follows creatures with an Intelligence Score lower than 5, who will migrate based on the seasons, primarily for climate, availability of food, or appropriate mating grounds.
Then, those with sentience, like humans, tend to migrate not seasonally, but instead as a permanent change in residence. People generally like to stay still, and build communities. You see human(oid) migration mostly due to refugees fleeing danger, of either natural disasters, or violent political situations.
Lastly, some humanoids are instead nomadic, choosing to move locations often, either due to following the migratory fauna, like Hunter-gatherer societies, or people that are refugees, and are politically not allowed to fully join certain societies, like the Romani ethnic group.
All three of these can be an interesting part of D&D ecology.
The monsters that I would think are least likely to Migrate are dungeon focused monsters, unless you go all the way to the Underdark, which might have it’s own climate, which could change, but not based on the surface seasons.
First there are flying monsters, like real life birds, giant versions of those birds, Harpy, Hippogriff, Griffon, Cockatrice, Roc, Peryton, or Aarakocra, which might be more humanoid and stationary. All of these could follow a basic North in the Summer, South in the Winter pattern. Is there a hunting season, where the local government will pay to kill these aggressive creatures when they’re around, like South Carolina, where you can get paid $75 per coyote you turn in. Or is the hunting season where hunters have to pay, like in Wyoming where residents pay $57 to be allowed to hunt Elk.
Then, there are swimming monsters, where certain creatures might swim from salt water to fresh water, or vice versa, for mating season, which is usually springtime for fish, and winter for whales. Does this mean that fishing villages have a boom during the Spring? My biggest question, what about the Kraken? Do people keep track of the Kraken’s migration, and then certain trade routes are inactive when it’s nearby? If one nation wanted to invade another, and they used ships, would they know that the Kraken is “in season” and accidentally get decimated before they even hit the shore?
I saw an idea that a safe, but slow, method of travel across an ocean would be on the back of a Dragon Turtle. Perhaps there is a Dragon Turtle that makes a season-long journey from one coast to the other. Maybe the way to ensure that it doesn’t dive underwater is to bring along with you lots of food that you throw in front of it, so it can eat right off the surface.
Dragons, and other creatures with lairs pose an interesting thought. If their primary source of food is migratory, then they too would become migratory to follow. Either that, or hibernate during the off-season. Do Dragons in your world eat food that doesn’t migrate, such as livestock, do they have a Winter-Lair and Summer-Lair, or do they hibernate for several months out of the year like a bear would? All of these offer cool adventure ideas.
Then, we have the refugee migration, which can be a good way to introduce large-scale disaster, without putting your players in the middle of it. Maybe there is an invasion of undead by a Lich to the East, and so thousands of refugees are making their way into your party’s home town, and it’s causing a variety of issues. Then, your party can slowly make their way East, dealing with both the increase in refugees and overrun villages, but also as they slowly get closer to the threat. This is a natural amplification of danger, so that by the time they get to the Lich, they’ve basically fought through an entire army, and joined the resistance fighters.
Lastly, we have the nomads of your world. Again, nomadic communities usually come in one of two flavors, Hunter-gatherers, or political/cultural outcasts.
In my setting of Emirace, goblins and orcs have lived in little non-permanent tribes, as they have been Hunter-gatherer, or the more violent version, raiding and pillaging. However, the Human Kingdom of Calder has never been weaker, and people are starting to consider whether or not these outsiders, who can speak, and craft goods, should be treated this way. Some goblins and orcs are attempting to join Human/Dwarf society (Elves have their own thing going on). Some larger cities might have opportunities for goblins and orcs, however smaller towns tend to be small minded and will refuse. Perhaps there might even be certain tribes that try to engage in the local trade ecosystem, because the fastest way to get respect, is to have money.
Also, in the Eberron setting, the nation of the Talenta Plains in Khorvaire is the home of nomadic Halflings, who ride dinosaurs and worship their ancestors. While they live in grasslands that could grow wheat and crops, they culturally just choose not to. They have access to technology and trade that the rest of Khorvaire does, they just would rather live on dino-backs and carry big spears.
Then, as a direct allusion to the Romani peoples, in The Curse of Strahd there are the Vistani, a nomadic ethnic group that are known for travelling between different planes of existence.