FOR MARES: A clear and plain NO!!!!  Mares are not allowed to wear chaps, nor jock-straps.  Stallions, on the other hand, can wear chaps or jockstraps.
All mares have the same dress code, naked and hereby all have the same initial, starting position.
The background thought behind having the same initial, starting position with all other mares (i.e. by presenting one’s self fully naked), the mares should entirely focus their undivided attention to fulfilling the very basic instincts of life.
The Horse Fair Fuck-Market is not a fashion show. The mares should convince the stallions not by wearing fashionable designer clothing, however, by willingly and enthusiastically employing all their natural talents and skills on providing great sensual pleasure.
(See/refer to the rules, rule no. 2)