My friend I am just a learner, so I am not an authority. But from my experience I can tell you that when 'strong' comes after a number, as in your example, it merely emphasizes the number. For example, the fortress was besieged by an angry mob of 5,000 strong men and women or Genghis Khan's army included 1,50,000 strong cavalry apart from 2,00,000 foot soldiers. So, your sentence suggests that this particular group of zebras with 15000-20000 members is relatively or surprisingly large! I hope your query is answered.