If the females in the harem have recently given birth, the new dominant male may kill their foals to eliminate any traces of his predecessor, and to bring the females into oestrus in order to further his own genetic legacy.

Do Stallions Kill foals?

Though mares do sometimes sneak outside the harem to mate with other stallions, on average the foals in a rival's band will not be sired by the new stallion. So if the new stallion kills them all, he might be killing a few of his own offspring as well, but he will primarily be getting rid of a rival's children.

Why do zebras kill foals?

Male zebras regularly kill foals sired by other males in order to assert their claim to be the dominant male in the herd and ensure they do not become a future rival. The killing of a female's young also frees her up to be a potential reproductive partner.