Generic term for animals considered en masse. What are you fishing for? Trout. What are you hunting? Boar. What do you see on the plain over there? Zebra. What do bison do in winter? They migrate south.
Well, sort of "wild animals" that you might want to shoot in the name of sport, not the cute fluffy kind that you don't want to shoot (unless it's that one that keeps defecating on your lawn, of course).
Because most of the time hunting terminology does not apply. For animals that are hunted, or fished especially, the singular is used for the plural in other cases. "I ate two mackerel today" is fine, for example.