Much depends upon the specific animal in question, the fertility of the land, the weather, and the regional climate.

However, in the US Midwest, using modern agricultural techniques, in a typical year, and using high intensity farming, hobby farm enthusiasts estimate that it takes 1 acre of farm to support each person (estimate at 100 kg) for a year.

I realize this doesn't actually answer your question but it should give you an idea on the lower bounds of acreage required to support a creature.

Probably a better method would be to estimate the bison population of the American plains prior to major European settlements, multiply by the average weight for the bison, and then divide by the area of that region.