Not all unicorns need a long, fantasy-inspired tail, but a horn is a requirement! If you're creating yourself a unicorn custom, this is how I personally sculpt horns.
Full disclosure: there are some sculptors (such as Maggie Bennett, the artist behind Breyer's Mirado) that sell pre-made horns, and I tend to use them often. But when I want the horn to be unique to itself, I’ve got to start from scratch. Also keep in mind that customs with pre-made resin parts, such as horns or wings, are not eligible to compete at the Breyerfest Live show. An advantage to sculpting one by hand is that it allows you to add in a wire for stability, which is harder to do with pre-made resin horns.
I like to leave the horn for near last when sculpting a custom, because even with a wire, they are the most delicate part of your custom, and when dealing with heavy prepping on other parts of the horse, you are very likely to break it when resting the model at awkward angles. As you'll see, I sculpted everything else on "BobbyCorn" except for her horn and her forelock, which will go around the horn.
Use your dremel or drill to create a hole where you would like the horn to come out of the horse’s forehead. It is a good idea to mark this spot first with a marker so you aren't guestimating while drilling. On solid-molded horses like stablemates, drill approximately halfway into the head.
Next, insert your wire. This will be the middle of your horn and will stabilize it. I'm using a 1/16th" copper wire, which is pretty stiff but can be bent if you want to add a curve to it. Using really soft wire won't be helpful for durability or to sculpt off of! If you make your wire too long, you'll be able to carefully cut it down, but try to make it about the right height to start. For hollow Traditionals (like Bobby,) I just let the wire hit the bottom of the head inside the mold. Use super glue (original, not gel) and baking soda to instantly bond your wire into place. I like to let it then rest for about five minutes to allow the bonding process to be complete.
After this, instead of using another, thinner wire, like with the tail, I use apoxie for about the same reason; it's hard to sculpt directly onto wire, as it is too smooth. So, I take a very thin bit of apoxie (I use either Aves Apoxie Sculpt or Magic Sculpt) and twist it around your wire. You'll see it's a loose coil. This is about the same spiral I want for the end result of the horn. Using some acetone-based nail polish remover and a wet down finger or brush (use gloves if you are sensitive!), smooth the apoxie just a little – and very gently. When you are satisfied, set the horse aside and allow it to dry. I like to let apoxie dry overnight.
In your next session, sculpt in the rest of your horn, using the hardened spiral as a way to prop up the rest of the sculpting medium. If you have problems getting the pointed end as thin as you want it, or the rest of the horn to be the exact shape you want it, you can sand it later or even carve it. I've allowed this session to sit and cure for about an hour, before I sculpted in the forelock.
The next day when the horn is dry, I sanded it down to have a little bit more of a pointed end, and carved some more texture in with an X-ACTO knife.
That's it! There is still some slight prepping to do on this unicorn, but after that she will be ready for paint.