The use of a matthew walker knot in horse tack is not unknown. While looking for more information about the fiador I came across this from Wilipedia - Fiador (tack)

Construction
In North America, a fiador is usually made from rope or cordage. Materials used may include horsehair, rawhide, cotton sash cord, or nylon. Cotton or nylon rope of approximately 6 mm diameter is the most common material. It runs behind the ears, over the poll of the horse, then joins under the cheeks with a fiador knot, or occasionally a Matthew Walker knot.