Let's start by rephrasing the question. If it was legal, ethical (no females with fawns or kids, animal within range for your abilities, etc.), of sufficient size, and no extra cost to you, is there a type (species or subspecies) that you wouldn't shoot? If an animal presented a shot and your PH put up the sticks and says "Take him!" is there any kind of animal that would make you look at your PH and say, "No thanks."
For example, some people just don't like the look of the horns on hartebeest. Perhaps you feel that you've already shot too many of a particular kind of animal. Or, in your opinion, they are not sufficiently challenging to hunt?
If there is such an animal for you, why wouldn't you shoot it?
Their are a lot of African animals that I wouldn't shoot.