I mean, the horse would win if it didn't have to carry a rider.
Or if each human runner had to carry a horse.
I'll grant that the weight of a human does affect the horse (and note that these races almost never involve elite-level marathoners), but from what I understand, humans are the only terrestrial animals capable of traveling more than 100 km/day (I believe the 24 hr. record is slightly over 188 miles on foot), so if the distance is long enough, no non-human animal, burdened or not, is capable of matching a trained human, especially on a hot/humid day.
Je suis méchant.