Complex sentence: Small numbers of stallions are allowed to join the mares for a few weeks in spring or early summer--each stallion then gathers a "harem" of mares and their foals to form a larger group of up to twenty or so.
Split into simpler sentences: Small numbers of stallions are allowed to join the mares for a few weeks in spring or early summer. Each stallion then gathers a "harem" of mares and their foals to form a larger group of up to twenty or so.