I would have no idea where to begin making a horse halter other than the rope halters with red or green rope.
I am not sure how festive you want to look. Our local tac shop has a few seasonal halters around 12-18 bucks. How much you looking to spend?
http://ecx.images-amazon.com/images/...aL._SL160_.jpg Christmas Holiday Horse Halter Bridle Set | Horse Tack | Tack Accessories