Other than the plated scales, tough leathery skin, frilled head, horned skull anatomy and sinuous tail, mythological and folkloric dragons have very little in common anatomically with actual reptiles. They have MORE in common with the Felidae genus (cat family) and the Aves Phylum Chordata (bird classification).
Like a cat’s eye, a dragon’s eye has a comparatively large iris with a vertical pupil. This arrangement allows the pupil to open extremely wide and receive more light than that of a human eye.
A dragon’s legs are also decidedly nonreptilian, despite the scaly coverings. A dragon’s legs are positioned more or less directly under its body, in the manner of mammals. (Most reptiles’ legs tend to splay out to the sides, offering much less support and mobility than a mammal).
Lasly, a dragon’s four feet very closely resemble those of a great bird. Each foot has three or four clawed toes facing forward (the number varies, even among dragons of the same kind), plus an additional toe, also with a claw, set farther back on the foot and facing slightly inward toward the dragon’s body, like a human’s thumb.
A dragon’s resemblance to a reptile is literally only skin deep So the next time someone you know refers to mythical dragons as giant lizards, you’ll have the know-what to save a life.
Last night on a D&D adventure, our tiefling warlock was in the process of getting rammed by an undead rothé.
Tiefling: I see it coming and put my horns down, ready to ram it back.
Me (DM): Wait, you what?!
Tiefling (OOC): I wanna ram him back! I have horns and I’m gonna use them.
Me: Fine, you squat down, presenting your horns to the rothé. It misses, but locks horns with you. Your horns versus a powerful undead beast, the animal wrenches its horns away, cracking the tip of your left horn off.
Tiefling: NO! Not lefty! He was my favorite!! It’s my turn next so I cast agonizing blast and aim for his… (OOC) Wait, can I roll to find out which horn is his favorite horn?
DM: Oh my god…sure.
*rolls a nat20*
Me, head in my hands: If an undead rothé could favor one horn over another, this one would pick the right horn.
Tiefling: Yes!! I cast agonizing blast on his right horn.
DM: It hits with force damage, splintering the horn off at the base and falling to the ground.
Tiefling: Aha!! You take my horn, I take yours!
The rothé comes back for the tiefling, ready to ram again.
Tiefling: Uh…I put my horns down, ready to ram too.
DM: You already lost one horn, why are you doing this to yourself?
Tiefling: I need to assert dominance!
DM: The attack hits, but with his right missing and your left missing, it’s left locks with your right horn. It strikes your puny tiefling head, cracking the tip off your right horn.
Tiefling: What?! No!!
The battle ended, the rothé fell in front of the tiefling.
Tiefling: (OOC) Can I take his remaining horns?
DM: Do a strength check.
DM: You put your foot on the rothé’s undead skull, pulling its left horn with all your strength. Your foot falls through the skull but the horn comes loose…too fast. You pull it into your shoulder doing *rolls a d4* 4 points of piercing damage to your shoulder.
Animals with true horns, which are mostly ruminants like the ram and friends, which is also the classical demon look, have a core of living bone in the center of the horn. The outer layer is composed of thick keratin, but it is the shape of the living bone core that dictates the shape, size and direction of the horns. The living bone core is often much smaller than the keratin component.
The keratin has no nerves and little blood supply. The living bone core has a whole bunch of nerves, an impressive blood supply (it is bone after all) and if you break the horn too close to the skull then you will also have a big whopping hole into the frontal sinus.
This would be at least as painful as breaking one of your bones.
Horns can certainly heal, but they often heal in a not quite right manner. If you haven’t completely fractured off the living bone then the shape template for the new horn will be different. If your character has lost the living bone core, but still retains the germinal layer of cells around the base, then they can develop scurs.
A scur is like a remnant horn growing without a template. They often occur when de-horning hasn’t quite been done right or after trauma. They have an unpredictable shape, can grow in any direction, and are frankly quite annoying.
Rhinos do not have a living bone core in their horn. You can cut off parts of these horns, they’re made of keratin and can be thought of similar to a very fancy finger nail.
But uh, don’t be tempted to do it like the poachers do it, where they cut a straight line including both horns and part of the skull. That is going to be the equivalent of fracturing a true horn at the base and entering a sinus (or nasal cavity in this case). I’m not posting those pictures on here.
But lastly, another anatomical feature we humans sometimes think of as ‘horns’ are antlers.
Antlers are dead bone with no covering when mature. They are shed every year. When they are mature they have no feeling and no blood supply except at the very base. While they are growing they have a good blood supply, but when mature they are inert. Antlers don’t grow bigger as such, they are shed each year and regrow, sometimes into a bigger or more pronged shape, depending on the species.
So take your pick. I don’t know which sort of horns your demon has, but I hope that’s answered your question.