It will come to no surprise to those that know me that I am absolutely loving the current interest in all things fantasy, both with Breyer's own OF line, and the fantastical customs being created by artists!
The only actual requirement for a unicorn is a horn. There are a lot of other modifications that people think of when they think of unicorns - including cloven hooves or a long "lion tail." I personally tend to not change their hooves, but I've always loved sweeping, long, expressive tails. If you'd like to give your own customs one of these tails, this is how I do it! Remember, there is usually more than one way to get similar results, but so far, this has worked the best for me.
I first start out with my future unicorn 99% prepped and customized in all other ways, except her horn and her tail. Because these two components are the most delicate, I'd rather have the bulk of sculpting and prepping out of the way. You'll want to use a dremel or drill to insert a hole at the base of the tailbone, if taking off the tail did not leave you with a hole already. Except for Stablemates, Breyer horses are hollow. If you're working on a Stablemate, this tutorial is the same, except make sure you drill about two inches into the horse to have a place to insert your wire.
For these tails, I like using a twisted wire method. I purchase most of my wire supplies in person at local hardware stores - I think it is really helpful to be able to feel the wires you're purchasing to figure out just how bendable they are, or if they are too soft. I use a 1/16th copper wire for my tails. You want the wire to be long enough to go about halfway into the horse when you insert it.
When you have the wire where you want it, use a liberal amount of baking soda and super glue to create "soda glue," a near instant resin-like substance that will harden and keep your wire in place. I then take a softer wire and wrap it in a spiral around the copper wire that remains on the outside of the horse. This will give your apoxie something to grip to when you are sculpting. Remember not to bulk this up thicker than the actual circumference you want to the tail to be! You can add a dab of soda glue at the ends of this wire to hold it in place.
Next, you'll create the base for your tail-tuft template. I like to create what is basically "tape paper" by making a square of my painters tape, flipping it over, and lining it with more tape so there is no sticky surface.
Draw the outline of what you'd like the tuft to look like, and carefully cut it out.
After you've decided exactly where you'd like it to be, again use soda glue to fasten it in place. I like to have the end of my wire tail at least a few inches into the tape template - you'll see I actually soda glued a bit more wire onto my tail to hold shape.
Now it's time to use your sculpting medium for the first time. While there are other two-part self-hardening apoxies out there, the only ones I can vouch for personally are Aves Apoxie Sculpt and Magic Sculpt. I like both of them, and either will work for this project. In this tutorial, I am using Aves (in blue).
Make sure you have the tail where you want it, and from the top of the tail down to the tuft, add your apoxie around your wire. It'll come out a little lumpy; smooth it the best of your ability with your fingers and acetone-based nail polish remover. It's not going to look perfect right now, but the goal is to eliminate cracks in the apoxie, as well as not have any divots that are thinner than you want the tail to end up being. This is also the point where I sculpt the hair I want at the base/dock of the tail. When you're satisfied, let the tail dry overnight.
The next day, sand down the tail as smooth and evenly as you can. You can take an X-ACTO knife and shave down bulkier areas to save some time. After that, it's time to take out the soda glue again. Decide how you want the tail tuft to flow, and "freeze" your tape template where you want it to stay, bit by bit covering the whole thing with a thin layer of baking soda and super glue (on both sides.)
I've found the rest of the tail tuft sculpting needs to be done in stages, to allow the portions to harden before you work on the thin areas of the ends of the hair. Depending on your own way of working, this may take more or less days/sessions. Sculpting the hair here is the same way I sculpt regular manes and tails. I start with "noodles" of apoxie in varying sizes, and then use rubber tipped sculpting tools dipped in nail polish remover to sculpt in bulk areas of hair.
Allowing the apoxie to set for about 30 minutes and then working it a little more for added details and layers is also beneficial! Before I'm ready to let the portion of hair dry all the way, I take a brush (usually a half dead overused brush I no longer want to paint with) and wet the whole thing down with nail polish remover, to help smooth the hair out.
Once you've completed the last of your hair, do another once over for minor prepping and sanding. Congratulations! You've completed your new fantasy equine tail, and it’s ready to prime and paint!