Final answer:
A horse responds to leg cues from a rider through its sense of touch, as their musculature is highly responsive to tactile stimuli.
Step-by-step explanation:
The horse's sense that makes it respond to leg cues from the rider is its sense of touch. The musculature of a horse is highly responsive, and it can feel even slight pressure from a rider's legs. This sensitivity allows for communication between the rider and the horse, enabling the rider to cue the horse to move forward, turn, or adjust its pace. In training, horses learn to associate these tactile cues (leg pressure) with certain actions. The coordination and training of a horse to leg cues are essential elements of equestrian skills, as illustrated by the disciplined movements of horses in various art forms and literature, where their responsiveness and fluidity of motion are often depicted.