This is a large dark blue-grey marble Galloping Horse that was found in the Civil Basilica. The horse wore a gilded bronze saddle cloth and was ridden by a naked heroic your of which only one thigh survives.
It is believed that the rider was being pulled off his horse by his hair by a figure that was standing by the horse. The subject was the Trojan prince Troilos who was pulled off his horse by Achilles and killed.
In the fourth century AD, the statue was extensively repaired: the front legs and parts of the mane were carved anew and in different marble. (from the description in the Aphrodisias' Museum)