From what I have heard over the years, hunters tend to classify game as "tough" if it tends to travel a long way after imperfect hits. I would put both elk and pronghorn in that category.
Or good hits too, that would be my definition. It also may have something to do with what PG pointed out, a pronghorn that goes 200-300 yards after a hit may well die in sight and will most likely be easily recovered. An elk that goes that far in the timber may not, leaving the shooter thinking it got away.
