Reducing grazing intensity is widely recommended as a strategy to increase grassland biodiversity through greater sward heterogeneity. Horses are playing an increasing role in the management of permanent grasslands across Europe, but the effects of horse grazing under contrasting stocking rates have been poorly studied. Here we describe the effects of two contrasted stocking rates (“High” 1.8 LU ha−1vs. “Moderate” 1.1 LU ha−1) on sward structure, horse foraging behaviour and performances, and pasture biodiversity in a mesophile grassland of central France. Horses selectively grazed vegetative patches of high nutritive value, especially at the moderate stocking rate (p < 0.01). This enabled them to maintain diet quality (diet dry matter digestibility: 59% DM) and performances (daily liveweight gain > 270 g animal−1 d−1) at the same level in highly and moderately grazed plots despite contrasted herbage biomass and quality. Horses, with their two sets of incisors, created and maintained patches of short grass in a matrix of tall vegetation in both treatments. Consequently, sward structural heterogeneity did not significantly differ between highly and moderately grazed plots, and there was no detectable effect of grazing intensity on floristic and arthropod diversity over the four-year study. The two stocking rates did however result in a divergent evolution of legumes for which abundance increased at the high stocking rate (p < 0.05). Abundance of Carabidae and grasshoppers from tall grasslands was higher at the moderate stocking rate (p < 0.05). We conclude that decreasing stocking rate would only have a marginal effect on grassland biodiversity, while farm performance will be strongly affected by the decrease in the number of horses per unit area.