Put a fence around the entrance to the zoo so that the entire zoo qualifies as a cage. Then, cover the whole zoo in savana grass and add plants and lakes. Place some animals then walk around as a zoo keeper/guest. To do this faster, do not get staff, food/water dishes or shelters. It will look unnatural. If you put gazelles in a pack, only put about two to six of them or some might ditch the pack. If you encounter across a dirty animal, wash it or all the animals will get sick and die. Note: Plant eaters will get eaten.