Diptera: Calliphoridae
Calliphora stygia
Within a couple of minutes of horses passing (in more ways than one), a deposit was being enjoyed by a number of flies, most notably Eastern Golden-haired Blowflies.
Had the riders, yet two hundred metres distant, turned around, they may have seen someone laying on the ground apparently photographing a pile of horse poo. If they did see, they obviously thought it better not to investigate.