Not enough time to throw together a costume?

Calm down. Fifteen minutes is totally enough time.

Can also be used as a base over which to sculpt more elaborate things.

Time: 10-20 minutes, depending on horn shape complexity and overall size.

Skill: Some.

Cost: Not much.

We're playing with hot things and creating weird fumes. Take the appropriate precautions.

Step 1: Tools & Materials

Required

Block of polystyrene (good solid stuff, not that compressed flaky crap that fragments into a mess of little pellets when you handle it).
Hot-wire knife (bought mine at a local craft store, but of course there are plenty of Instructables on how to make one).
Respirator or well-ventilated workspace, or both.
Theatrical adhesive of choice (spirit gum, latex, etc.)

Optional

Utility knife
Felt-tip pen/marker
Small heat gun (butane torch with a heat gun adapter works well).

Step 2: Slice and Dice

Or just slicing, really.

Depending on how large the initial block of polystyrene is in comparison to the hot-wire knife, it may need to be cut down a bit before any detail work can begin. My block was too thick for my knife, so I had to cut it in half first. To do this I heated up an old utility knife blade with my butane torch and used that to slice the block to the desired size.Use an old, crappy blade for this -- it doesn't have to be sharp at all, as it's the heat that does the cutting -- because it will get covered in bits of gooey plastic, and they will never come off. Wear a respirator and/or get good airflow going. Plastics make some lovely fumes when they melt. Don't be breathing that noxious stuff in.

Step 3: Base Shape

Grab a marker and draw out the basic profile of the horn. Alternately, skip straight to cutting and just freehand it all. Copy the result over to the other horn and repeat.

Fire up the hot-wire knife. Press lightly and let the heat do most of the work; don't rush it by forcing the knife, or there's a good chance the thin wire will snap. Precision isn't a high priority at this stage so don't worry about not following the lines too closely.

For matching horns, trace out the completed profile onto another piece of polystyrene, then cut that one out too.

Step 4: Secondary Profile

Draw out and cut the other profile view, copy to the other horn and cut again.

Step 5: Rounding It Out

Looks kind of squareish and blocky and not very natural, doesn't it? Time to start shaping it. Cut along each edge to go from a square to an octagon. Try to make it one continuous cut along the whole edge because it's tricky getting it smooth if there is constant breakout and restarting. Keep trimming down the edges. The horns should be getting rounder with each pass.

Once the desired shape is achieved, press in the bottoms of the horns a little bit. This dent helps them sit more flush on the forehead so the adhesive has a larger contact area.

The spirals in the unicorn horns were made via very careful application of a heat gun. Lightly melting the surface also seems to make it a tiny bit more sturdy and less squashable.

Some paints and markers probably work on this plastic, but offhand I don't know which. Do not use spray paint, however. It dissolves polystyrene.

Step 6: Dress Up and Goof Off

There are so many possible shapes that can be made, and many different configurations in which to wear the horns. They are very lightweight. I often forget they're even there, until I go to push my hair back or I lean down to grab something and whack them into a wall or against a table. I've always used spirit gum for my horns, and it takes several horns-meet-wall-or-doorframe incidents before they even begin to come loose.

Follow the directions on the adhesive -- do an allergy test first! -- and apply horns to face. Proceed to terrorize the populace. Gore the first person to make a "horny" joke.