I'll have more information about real fire horses in a later post, but here's how I made the collar. The same concept could be used to make a normal harness collar- it would just be made in one piece, and open at the top instead.
First, I made a paper pattern and a felt copy to fit it to the horse. (shown here with most of the rest of the harness)
From left to right: 1. cardboard pattern piece with twine glued on (for padding). 2.thin cowhide leather pressed into and glued in the center gap. 3. leather wrapped over and glued onto the bottom 4. final piece bent into shape.
Same thing, from the back side.
The two pieces are hinged at the top with a piece of 1/4 inch leather lace (to fit under the mane, bottom) and one end of the back strap is glued onto that (top).
The inside of the collar is lined with chamois cut slightly smaller than the cardboard piece. I don't think the real ones were lined, but this covers the edges of the leather and protects the model. It is not visible when the collar is on.
Collar on the horse.
Front opening of the collar. This will close with a latch on the end of the hames.
Top of collar with cutout for the mane. The hames will attach permanently inside the collar's groove. They will be made of bent brass strips.
And that's the collar!