Ditto and Jack Demonstrating Brawn
These are two stallions of different harems.
It's essential that stallions demonstrate their brawn to other stallions, and so we often see these moments of strutting. The stallions carefully observe the other's state of health-strength-stamina. If weakness is sensed or detected, than opportunistic actions may take place by a rival, such as fighting until a band is taken over, or mares stolen, adding to the size of the harem of the victor.
A healthy stallion will never stop trying to acquire mares, nor will he stop displaying his brawn and might, to demonstrate how adept he is at protecting what he's earned and what is his.