What you need depends on the breed of horse showing, the discipline and the show level. Basically, a stiff brush to remove dried dirt or mud, a soft brush to polish the coat, a rag for boots and tack, a rag for the horse, rubber bands or yarn to fix braids/ banded manes or tuck in tack keepers, hoof oil/polish, coat polish, spare safety pins, a bucket of water for the horse and a drink for the rider. A pad of paper for copying patterns and a copy of the showbill are good ideas too. Good luck