A few more points for this interesting thread:
1. Yes, Cape buffalo are very rare in American zoos. So rare that, by having them, the Jacksonville Zoo is America's ONLY zoo to exhibit all of the famous African "Big Five" -- Cape buffalo, elephant, lion, leopard, and rhino.
2. In just over a week, I'm going to see another species that qualifies for this thread, WHALE SHARKS! I'm going to see them at the amazing Georgia Aquarium. Never been there before, so I'm quite excited.
3. The Berlin Tierpark (sometimes called the East Berlin Zoo) probably displays many of the rare animals discussed on this thread, and it's the only zoo I've seen that has such a "complete" collection of many animal types. Examples: (a) they have spotted, striped, and brown hyenas; (b) both Sichuan and Mishmi takins; (c) All 4 types of llamas (llama, alpaca, vicuna, guanaco) in side-by-side large fields; (d) I believe they had 3 types of zebras in similar side-by-side pens; (e) much more!