Spell casting is generally considered a common, natural thing in Dungeons and Dragons. Wizards cast spells that they learn and study, Sorcerers cast spells from the innate will within themselves, and Bards cast spells from being sexy (just kidding!). Magical creatures cast spells too.
For all the ways that magic is cast in DnD, very little is said about who and what can cast spells specifically. Surem we know a little bit about the casting differences between Wizards, Warlocks, Sorcerers, Clerics, and Druids, but nothing in the rules establishes hard limits that would say how magic is restricted in any given DnD world.
All of this begs the question: can wild animals cast spells?
On The Nature of Spells
Spells in DnD have a very poorly defined border. The primary rules about spellcasting in general limit spell uses per day depending on their level, separate spells into types, and establish a framework for spell attacks. As far as can be inferred, there is nothing that limits spell casting to specific classes. We can see that some creatures innately possess magic abilities, and these have their own limits, but we don’t know where these creatures get magic from or how they cast.
Presumably magical creatures could learn more spells the same way that player class characters can, but this isn’t something that comes up in many games. This idea is ostensibly up to the DM to decide in their own game when crafting encounters. Choosing spells for enemies might be overlooked in some games, but it really is a vital part of fine tuning challenges for your players to overcome.
Taking things a step beyond magical creatures, what about mundane creatures?
If a magical creature can blast off cantrips willy nilly, why couldn’t a squirrel do the same?
Magical World vs. Magical Creatures
Depending on how the DM considers magic to work in their world, creatures may or may not have access to spell casting abilities.
If magic is something that is part of the world and spell casting creatures tap into that then there is no reason that this logic couldn’t occur across the board.
If magic is something that is an innate quality of a creature, then only creatures with magic in themselves would be able to cast spells.
The idea follows that the source of magic would matter for the argument. Perhaps magic is an additional sense that a creature has to have to be able to cast spells, or maybe magic requires a certain base level intelligence. Magic could take a certain fortitude to channel it.
While these reasons could be true since there are no rules that prevent these limitations, it falls flat with a quick flip through the Monster Manual. There are creatures that have low intelligence that are able to cast spells. There are weak creatures with magical abilities. There are even “normal” creatures that we see spells given to – humans notwithstanding. So with all this in mind, it makes more sense that magic is, in a DnD 5e mindset, part of the world.
If magic is part of the world then mundane creatures are highly likely to be shaped by it. Mundane creatures are perhaps even more likely to be affected by it.
In any given environment that organisms inhabit, their interactions shape how those creatures adapt and evolve over time. The interaction between organisms is the specific science of ecology. Ecology is a great field of study for Dungeon Masters and writers as it is really the story that a collection of lifeforms tell when you look at them together.
A great example of this is in the classical evidence of evolution around Darwin’s finches. Like most historical figures, Darwin is no saint and certainly no hero, but his work to establish evolution as a concept was sound. Darwin’s finches show clearly the effect of ecology on isolated species. These birds, separated by islands, had dramatic changes in appearance based on available food sources, cohabiting predators, and many other factors.
The primary data collected examines the changes in the bird’s beaks in relation to their food source. The better suited a bird’s beak was to the food source on a particular island, the better it did and the more offspring it had. With each generation the best traits survive to have offspring that are more specialized for the environment they are in and presto-chango the birds adapted over time.
Now this is the first part of the process of evolution, but it is not evolution. On a long enough timescale it can be. As the birds remain isolated they would eventually look more and more different from their common ancestors until an onlooker would eventually say that they are not the same bird.
Let’s build on this real world concept in a magical environment. If beak shape gives otherwise mundane finches an advantage in an environment, would rudimentary spellcasting have similar effects?
Let’s take a bunch of shrews for example. We’ll start with our first population of extremely normal shrews on a central island in a chain of islands. These shrews are well adapted for their environment already. They’re good at dodging predators for the most part, they blend in with their surroundings, they’re decent swimmers, and they can eat a variety of foods to survive. One day however, a huge wave crashes into the island and sends shrews far from their home. Survivors of this shrew tsunami end up as groups on the surrounding islands where they washed ashore. They’ve been spread in all directions.
What kinds of differences might occur over time in these populations as they adapt to their surroundings?
The shrews flung to the north are in a colder climate. The island is more mountainous and the warmer beach offers little food. In normal circumstances you might suspect that our creature would grow thicker fur and over a few generations migrate further up the mountain, but we’re in a magical world!
These shrews instead learn how to breathe fire.
How? They saw other magical creatures do it in the chilled landscape.
Shrews that could learn to breathe fire develop a serious advantage over the ones that cannot. The fire can keep them warm, root out prey, cook food, and more. The environment only adapts slightly to the presence of the shrew, as we’ve established there are other fire breathing animals on the island already and the plants that have co-evolved with them are lush and moist to resist forest fires.
Perhaps there are some trees further up the mountain that rely on routine forest fires to fertilize the land and help the next generation of tree grow. All of this is possible because we have fire breathing creatures.
Our shrews in question would likely also increase in size over time without other major predators around. When they first arrived nothing on the island ate shrews, since they were never there before, so for a bit they went unchecked. This is what allowed them to gain their initial foothold.
Over time predators adapted to eat the creature and balance out the ecosystem. Another reason these fire breathing shrews would increase in size is because cooked food yields far more calories and nutrients than raw food does. This is one of the reasons humanity took off on earth- we learned how to harness external energy to better survive tough conditions.
So we have one magic shrew in the north as a rudimentary example, but this is only the beginning. Let’s imagine that some of our original shrews ended up on an island that is a shrew paradise. No predators, abundant food, the perfect temperature. In this situation we would expect the shrew population to explode and we’d also expect their visual appearance to expand in diversity. If the primary factors for a shrew’s initial coloration were survival, without the presence of predators we could see color variations, different fur types, and a lot of other things.
In this ideal shrew island, where the universe allows for magic, we can also assume that the shrews might develop spell casting abilities similar to the fire breathing shrews in the north. The difference here is that when the shrews have no predators the primary competition is between individuals. Shrews that can show more impressive displays, provide more food, or better fight off rivals will breed. Magic can help with all these things. These shrews may even develop magical specialties all of their own.
We could see them diverge into altruistic paths where they don’t fight each other but instead learn to craft showy visual spells, like learning Prestidigitation that allows them to change their color at will. In a more aggressive society we could see them learn basic combat magics of all types. In a culture that does not favor rivalry we might see them develop casting skills that focus on building dens or shaping the landscape to be even more favorable for their own kind.
These are just a few examples of how magic is a rather extreme overlay on evolutionary biology, but it could exist in nature. Any DM can extrapolate this idea out into their game.
Your Magical World and Squirrels With Spells
As a DM you get to decide how prevalent magic is in your world. It could be extremely common or extremely rare or somewhere in-between. In a world where magic abilities are common it makes sense that some creatures have picked up simple spells along the way. These could be as basic as cantrips, but as magic becomes more and more common the world becomes more and magical and creatures would know more complex and higher level spells.
These clearly will have an effect on the ecology of any given area, but if there is room for a magical niche of creatures then they will likely find it. Life… uh…. finds a way.
So How Does This Change Your Game?
There are a lot of ways that you can use this as a DM. If fish knew the spell Create Water you could find them existing in weird places. They’re no longer confined to the area they are in by the volume of water present. It would be much easier for fish to survive a drought if they could call forth water themselves.
This might make desert oases more common in your world. The water is continually replenished by the fish and over time the desertification around a few lush pockets of land gets you an area dotted with little pools full of fish.
This comes into play when your players are dying of thirst while trudging over sand dunes. They come across an oasis with fish, but they can’t spend their whole lives there. So what do they do? Simple, they take a fish with them. If the fish casts the spell whenever it is running out of water the party only needs a container that can hold water and a fish or two to cast the spell as they drink it. Sure, they have to drink fish water, but it’s better than dying of thirst in the desert.
What type of magic and how much any creature can cast is really up to you as a dungeon master, but there is a whole wealth of ideas you can run with. A dog that can cast mage hand would be a real handful. A squirrel that can cast invisibility is better able to hide food and avoid predators while also being a nuisance to your party.
There’s a lot you can do if you give everyday creatures access to magic. This compounds if you don’t make the creatures more intelligent too. A creature’s actions are more based on instinct and perspective, so if you think about what they want you can often find spells that are perfectly suited to just about any mundane wild animal in the game.