Unicorns, both ponies and horses, are one of the many species who have developed specialized body-parts or organs for controlling magic. In their case, a horn growing out of the center of their forehead. This is used by the pony/horse to directly control and amplify their internal magic, allowing them to consciously cast spells. There are many creatures with similar structures, but they all function slightly differently. So this will be specifically about how unicorns cast spells.

-The Horn-
There is much debate about whether or not they should be considered ‘true’ horns or not. On one hoof, they do follow the general criteria of a core of live bone covered by keratin. But on another one, unicorn horns do not serve the same function as other horns at all, and did occur completely independently. If one compares to, say, a cow’s horns, they will find that the bone core has a completely different structure and that the keratin cover is much thicker and sturdier on a unicorn. Colloquially, they are still called ‘horns’, and shall therefore be referred to as such here.

Construction
So, as mentioned. Unicorn horns consist of two parts; the core of bone and the keratin cover.

The core has a hollow center with a cranial nerve going through it. The rest of the bone is full of smaller hollows, surrounding the center, which are filled with tissue and microscopic crystals. Each of these is connected to the cranial nerve via smaller nerves. These crystal hollows are how unicorns can cast and control a wide variety of spells consciously.

Due to its porous nature, the core is quite fragile; even smaller cracks can lead to permanent damage to a unicorn’s ability to control their magic. Larger breaks closer to the base can even lead to brain damage.

Therefore, it is covered in a layer of keratin. It is very thick and sturdy, and can take quite a lot of force before cracking. It is usually colored the same as the pony’s fur, but that is mainly because ponies find it attractive. It is very possible for ponies to have differently colored horns.

The keratin cover is completely without feeling. A pony can usually tell if something hits it -the core is sensitive and vibrations propagate down into it- but don’t feel touch. Many non-unicorns get very disappointed when they go to kiss their partner’s horn, only for the partner to not even realise they’re doing it.

Any damage which does happen to the keratin is usually not dangerous. It can even be completely snapped off -as long as the core isn’t hurt, the keratin can be regrown with no lasting damage to the horn. The cover grows continuously throughout a pony’s life. Long ago, unicorns used to instinctively rub their horns against trees to wear them down; nowadays, files are preferred. If they are not filed down, they just continue to grow longer -until something inevitably hits them hard enough, and the tip snaps off.

That’s not to say ponies with longer horns all just let their keratin grow out longer -though that’s certainly a possibility- but the length of the core varies from individual to individual. Celestia and Luna’s cores are quite long, so their horns have to be so long whether they want to or not. Fire Flare’s core is average, but she likes the way a long horn looks.

Some horns curve back and over the head as they grow. As long as the curve isn’t too extreme, this isn’t actually dangerous to the core. In fact, there are some studies that show that the curved structure makes it more resistant to breaks. Curved vs non-curved horns come and go in trends, with straight horns being in fashion currently. There are braces some ponies wear in their sleep which forces the horn to grow either straight or curved.

Development
When a unicorn is born, the length of the horn varies from being a barely noticeable bump to a slightly larger bump. There have even been a few cases where the horn is so flat against the head, the baby was actually mistaken for an earth pony. In the few cases where the horn is longer, it usually results in serious problems; any larger than a bump and the horn tends to get stuck in the walls of the birth canal.

The horn grows really quickly after birth, and is showing the stereotypical conal shape after just a month (i.e. when they’re a “toddler” -ponies develop a lot quicker in all aspects compared to humans in that early stage). At this point, the crystalline structures and nerves have been developed enough for the foal to start using magic. Not sophisticated spells -the foal cannot even speak yet, never mind trying to focus and shape the magic into complex forms- but light and levitation is definitely possible.

The growth of the horn slows down a bit after that. But by the time the foal reaches about the same age as the cmc or slightly older, the core has grown to its full size proportional to the head. This is also when a pony’s magic has settled into its final shape (i.e. a pony has found their talent and gained their cutie mark), and the foal’s mind is developed enough that more complex spells are feasible. Please note that this is just the average -some ponies start learning spells earlier or later, and the core might be proportional earlier or later.

After this, the growth of the core slows to a crawl and only grows to remain proportional to the pony’s head. The keratin still grows though, and might begin to curl slightly as it reaches beyond the core. It differs from individual to individual. For some, the curl actually starts before then, back when the core isn’t fully grown, and usually requires some kind of adjustment; if the curl is too pronounced and early it can warp the core as it grows. In modern times, that isn’t a big issue -the pony just has to wear a brace during their sleep to straighten the base out.

Once the pony reaches adulthood, the core stops growing completely. The keratin still does however, with varying speeds from pony to pony. Some have to go file it down every few months, some accidentally gets a chip in it and spends the next few years yelling at it to grow out longer so they can file the embarrassing nick away.

Damage
If the core of the horn is damaged, it can cause significant hinderance to a unicorn’s ability to perform magic. Damaged crystal hollows are non-funtional to their purpose, and if the nerves are exposed, channeling magic is painful. As is cold, or wind, or heat. In general, exposed nerves are not good.

In cases where the damage is small, this is usally partially fixed by itself -the keratin grows up and covers the exposed or otherwise damaged part. It is still painful and more difficult to cast, but the pony can mostly live a normal life without worrying about things such as infections or further nerve or brain damage.

In the cases where the core has been snapped clean of, it’s a little more complicated. As the keratin grows past the edges of the bone, it will naturally start to curl inward -down into the exposed nerves. So, any pony with a snapped horn has to be very very careful to file the keratin down every so often. And since the keratin therefore does not cover the exposed nerves, they are affected by the elements and suseptible to infections. So, an artifical covering is often worn; usually metal lined with fabric, but some softer plastic-like substances are used as well. Or if they’re Tempest and want to still look cool and be able to shoot explosions ut of their horn, they just clean the area often and ignore any winces of pain.

In most cases, any damage to the core is permanent; once it’s is fully grown, the it has next to no ability to heal itself. If it happens before then, to youger foals, it can grow out and over smaller nicks or cracks. But even then, larger breaks are permanent.

Social interactions
Horns play some role in unicorn social interactions. Knocking them together lightly is playful and affectionate, and parents often do it with their children. Knocking them together hard is a challenge, and a very aggressive action due to the risk of cracks. Resting them against one anothers foreheads is very affectionate and intimate. As mentioned, stroking or kissing the horn does nothing as the pony cannot even feel it. It’s very rude to touch a pony’s horn without consent -especially when they’re performing magic, as it can cause the spell to be disrupted.

Ponies don’t usually actually use their horns for fighting; it is quite dangerous to actually hit something with them, since they both risk breaking, and if you hit it straight on, all the force is centered directly on your forehead. It’s a recipe for brain damage. Still, a unicorn will often lower their head when they’re afraid or want to look intimidating -they’re not going to charge, but other creatures do not need to know that. And any pony when pushed far enough will fight with all available tools -including unicorns ramming their horns into things. Yes, it may permanently injure them, but in a life or death situation that may be the preferable option.

Decorating the horn does happen, but it’s a bit tricky. Anything put on it will interfere with their magic to varying degrees, as it disrupts part of the magic aura, depending on area covered and the material of the item. So it’s not very common for ponies to do in day-to-day life, aside from a few who deem the aesthetic more important than the loss of control. Many older unicorn rituals involved covering the horn completely in metal (making them unable to cast anything); such as many older marriages where each partner in turn had their horn covered and some actions performed -thus proving their trust in their new spouse. In modern days, this tradition lives on in the horn-rings most unicorns choose as their marriage token.

-Spells-
This adaptation which allows them to more directly control their magic has come at a cost. While many other species -and the other tribes of ponies- have some type of passive magic, all of a unicorn’s magic is direct. They have to think and actively perform it. The action of shaping their magic into performing specific tasks is usually referred to as ‘spells’.

When a unicorn casts a spell, their horn lights up with their magic aura. The color varies from pony to pony -since both the cutie mark and the color of the aura is based on a pony’s inherent magic, they usually share colors. Not always -for example, Sweetie Belle has a green aura, but no green in her mark- but it’s quite uncommon.

Basic Spells
The most basic spells there are, is “Light” and “Levitation” -so basic, they do not even have fancy names. Light is just what happens whenever a unicorn activates their magic. Push more energy into it without actually trying to do anything = more light. Levitation is more complex, as it requires the pony to extend their magic out from themself and envelop something else. But it is somewhat instinctive, and many ponies can even manage to do it as toddlers. Surprisingly, it actually gets more difficult when the pony grows a bit older; they start to try and think about what they’re doing and not just doing it.

Some might argue there needs to be a third spell in the category of basic spells -”Explode”. The opposition being that it’s just a derivative of Light. If a pony keeps pushing even more magic into their horn without doing anything with it, eventually it explodes into their faces. Literally.

Specialized Spells
Aside from these, what spells ponies learn varies a lot. Technically, any pony is capable of performing any spell, provided they have enough magic stored to cast it. Technicalities isn’t reality though, and most ponies only learn a few spells in their life. In general, ponies find spells that relate to their talent a lot easier to learn than those that do not. The reason is that each pony’s magic has a certain ‘shape’, which is affected by their experiences during childhood. This shape then makes the cutie mark appear as a certain image. So each pony’s magic is shaped as their talent -and it is therefore easier for ponies to cast spells that align with that. The magic naturally ‘wants’ to do it, after all. The more different a spell is from a pony’s inherent magic, the more a pony has to concentrate and force it to do what they want, rather than just letting it take a form it already wants.

So for example; Rarity knows spells for finding gems, minor transfiguration (altering the shape of a material, not what the material is), and some very shaky weather manipulation magic (which she can only half-remember from her time with Rainbow’s mark). Two of these are tangentially related to her talent, so they were very easy to learn and easy to perform. But despite remembering how to make a cloud rain, she still fails half the time due to how difficult it is for her to actually do -she rarely has the concentration required for making her magic to something so different from what it’s naturally shaped for.

Twilight and Starlight are examples of a special case. Twilight’s and Starlight’s talents are for magic. As such, their magic is extremely malleable and they can more easily shape it however they want. Thus allowing them to learn a lot more variety of spells. But in return, their spells are less naturally powerful than a pony who’s talent aligns with the spell. So, Rarity using the same amount of magic to power her gem-finding spell as Twilight would get more accurate results over a larger area. This isn’t an issue for Twilight and Starlight specifically though, as their internal pools of magic are large enough that they can just pour more magic into the spells to get the same result.

This is also why ponies are impressed by Trixie’s magic, despite one third of the population being able to use magic. It is not that she can cast spells, it’s that her illusion magic makes it appear as if she can cast a wide variery of spells with next to no effort.

Horn damage’s effect on spells
As mentioned, if the core is damaged it causes issues for the pony trying to perform spells. As long as the cranial nerve isn’t too damaged and there are sufficient crystalline structures, it is still possible. But it is painful (damaged nerve with energy running through it -ouch), and takes a lot more magic and concentration to perform spells.

But if it is too severe, such as in the case of Tempest’s horn, performing spells is impossible. The nerve damage is too great (by which I mean… the nerve is 90% gone) and the crystalline structures are all missing. She can still channel magic into what’s left, but she has next to no control over it. She can somewhat choose the direction of it, by physically pointing her head, and choose how much magic she uses, but that’s it. It pretty much does what it wants. Which, mostly, is exploding.

-Magic pools-
Every creature has an internal pool of magic they can draw from; but some use this to pull in even more magic from the environment and can therefore get away with smaller pools. Unicorns, however, almost always just use their internal magic -as that is the only thing they can manipulate with their horn. Unicorns draw from this pool and pour into their spells. In general, pouring more magic into a spell makes it stronger. The size of this pool varies between species and individuals. For ponies, the size of their pools are affected partly by genetics and partly by how much magic the pony uses.

For the genetics aspect; some are just born with larger pools than others. It is somewhat inherited by their parents, but not completely; some with smaller pools can have a kid with a large pool, and vice versa. It’s a mix between inheritance and luck.

Ponies can alter the size of their magic pools somewhat. If they do not use their magic a lot, it will shrink over time. Conversely, if they use it dry a lot of times, it will expand somewhat. There’s a limit to this, and a pony’s pool at its smallest compared to its largest is about 30-40% smaller. Again, varies from individual to individual. Twilight was born with a naturally large pool, and she uses her magic dry so often that it is about as large as it can get. So she’s on the very, very end of the normal distribution of internal magic pool sizes. Something which many others are more than a bit jealous of.