If we didn't have cattle, would we have buffalo? Large wild herbivores in general? (Hmm, results of a quick google... there were 30-60m buffalo in the 19th century, and there are 94m cows in the US today. Given relative size, methane output has probably been more or less static over that period.)

That's a good question and I don't know.

The net terrestrial vertebrate biomass has shifted markedly over the past century (and undoubtedly over the previous 50,000 years of the Human Diaspora). What the pre-Columbian American bison population was I have no idea.

Total pre-industrial wildlife biomass ~10,000 BCE, by estimates, was ~1/7 that of the present, and smaller than humans in total:

http://peakoilbarrel.com/wp-content/uploads/2015/04/Terrestr...

Some people think buffalo herds exploded when native Americans died off.

The herds may not have been that big pre-columbus.