Good grief! Horses and gunpowder, Elsie!
The horse arrived in the New World with the Spanish settlements in the southwest. We tend now to picture the Indian on horseback (at least the Western tribles like the Sioux and Cheyenne, less so the Eastern groups like the Algonquins and Iroquois). However, even in the western cases, their "traditional" lifestyle had changed radically only a few generations before.
Gunpowder was a European import as well. Prior to these innovations, the native Americans were very limited in their ability to kill buffalo, especially on the open plains where the herds thrived in the greatest numbers. Think about it. You're on foot. You have a bow and arrow. There's a buffalo herd over there. It's wide open country. You see them. They see you.
You and your buds do tricks like trying to slowly, slowly crawl up to them while cloaking yourselves in the hides of dead buffalo. Once in a while it might work. You're not going to have a big impact on a prairie that's loaded to capacity with buffalo.
Not much of an 'impact' on ANY of the large animals the poster I was responding to said the proto's wiped out.