What picture comes to your mind when thinking about the indigenous American Indian? Well, here's a factor you should consider.
There were horse-like creatures in our hemisphere as far back as 55 million years ago. These creatures were similar but not really horses as we know them. In any case, they went extinct about 10-12,000 years ago due to climate changes and, maybe, hunting by humans. There were no horses in the Americas from that time until 1493, when, on his second trip, Columbus carried a few here. The Europeans had developed the horse and found out how to tame, ride and use them for agriculture and warfare.
So understand, the indigenous people we called Indians had no idea what a horse was until the Europeans arrived. Tribes of the eastern US, east of the Mississippi, had no horses, or very few, ever.
The big influx of horses came to the Americas with the Spanish military conquest of Mexico beginning in 1519. The horses the Spanish brought and dispersed spread northward into the land now called the western USA. Escaped horses, we call mustangs, formed large herds on the Great Plains. It wasn't until the early 1600s that the Indians of the western and northern regions began acquiring and integrating these horses into their cultures.
The tribes that best conquered the use of horses were the most powerful. When the Europeans spread westward using horses to pull wagons, the Indians had no idea who they were. To them, they were the same as other tribes trying to take their land. Then, the migrating Europeans introduced them to the gun - but that's another story.