Sounds very similar to Cape buffalo. Some will run 50-100 yards with a decent chest hit from something like a .375 H&H and keel over, and other seem to be made of some other substance!
Once in a while even smaller animals act similarly. Eileen once shot a rutting pronghorn buck broadside through the chest with a 120-grain Partition from a .257 Roberts at 2900 fps, at modest range, and despite a major pumping of blood from the wound (much like the buffalo described above) the buck tried to mount a doe! So she shot the buck again, also a well-placed chest shot, and he staggered to the ground--but even then was making motions with his head, as if trying to hook her when she approached. She finally shot him again just to end it, and the buck was so bled-out by the time he quit, when field-dressed there wasn't more than an ounce or two of blood inside the chest cavity.
“Montana seems to me to be what a small boy would think Texas is like from hearing Texans.” John Steinbeck
