I created this design in Fusion360, based on this fabulous tutorial by Noe Ruiz.  I'm printing on a dual extruder Sigma BCN3D:

Modifications

Following the video will create a horn that's slightly too small to fit the neopixel ring inside.  Start with a sketch that's around 42mm across instead of 38mm across to be sure it will fit.

I also added a 4mm x 13mm hole near the base for the USB charger, and added tabs right at the bottom for sewing or for attaching to the mounting plate.

Since the on/off switch is on the Gemma, and that might be hard to get to, I created a slide-in mounting plate that can be sewn to a headdress or hood.  This way the horn is easily removable for charging or for turning on and off.

For non-dual extruder printers, here's an .stl file of the horn and spiral merged together.

This horn was printed with dual extruders for two-color goodness.  I printed the horn in glow-in-the-dark ABS and the spiral in purple ABS that changes to pink when it gets warm.  Magic!

To print the unicorn horn, the 3D printer will need to be capable of printing a minimum of 150mm tall, and the base of the horn is around 42x48mm, which should fit on most beds.

Slicer Settings

Download and import the .fff files below into Simplify3D.  These settings are for dual extruders, with 3mm ABS.  The horn will print with the left extruder and the spiral with the right.

Horn Print Settings

• 0% infill
• No supports needed
• No raft
• Skirt/brim can be helpful

Platform Print Settings

• 20% infill
• With suports
• With raft
• Skirt/brim can be helpful