As to the Gap Theory, while there is one in creationism, it's missing in animals. And, no, a one-lung pneumothorax will not necessarily kill and certainly not quickly on a wild, vital big game animal suddenly thrown into flight mode. Humans can walk around with partial pneumo's though they generally don't feel too well. While a hemothorax is a more sure thing depending on how large and the volume of blood lost. A pneumo- without a hemo- on a shot big game animal would be quite unlikely.
The cupola of the lungs run somewhat parallel with the lower edge of the spinal column as far as they go generally and the aorta runs parallel in there too with some anatomical variation in some hoofed animals. Gemsbok have the very long spinous processes that give them the very deep chest look. While arrows and bullets can and do slip through various solid body areas from time to time and may be lost, I'm convinced most, most, of these animals expire.
George, you probably know ungulates are different from humans in the pleura... we have one for each lung (essentially) but ungulates have both lungs in a single. An ungulate with a single pneumothorax will likely find it doubling soon (er) than a human.
I am sure you have heard of bullets passing through the chest without tearing lung tissue, the source of the gap theory. Arrows are very unlikely to do so.
