There was slavery and genocide in America before Columbus. And Spaniards accused Columbus of gross cruelty, and he was sent in chains back to Spain before the King of Spain released him.
Custer was a mixed bag. No rational person ever called him a coward, tho, and he fought and killed plenty of white guys in the Civil War. Unless you believed the movie "They Died with Their Boots On", there has been a TON of negative history written about Custer. By the time I started high school in the 70s, more folks attacked his reputation than revered it.
The Pilgrims did not move into an established town. Illness brought by visiting fishermen had killed out the town of 2000 Patuxet...almost 100% mortality. While that meant there weren't enough natives to overrun them, that doesn't mean they walked in and took over a town. In the first year, sickness killed half of the whites.
"Why couldnt the chinese have brought them to trade in the early 1400's ?"
I guess the big problem is that there is no record of any kind of horses in America before the Spanish. No paintings. No drawings. No toys. No words for horse. No oral tradition.
Nor is there any indication the Chinese were exploring America, let alone trading here. Nor the Koreans. Nor the Japanese. Either it was an enormous secret, concealed for all time, or it didn't happen.
Further, the theory requires the Nes Perce to have bred horses without any of their neighbors catching on to it.
It is kind of like the wheel. The concept wasn't entirely unknown prior to Columbus, but there is no evidence wheels were being used to transport anything anywhere in America.
Things just don't happen in a vacuum. If there had been horses, there would have been ripples in the historical pond - toys, drawings, words for horses, attempts to steal them...something.
If you want to hold to your theory, have a nut. If you want anyone to believe you, you'll need more than paintings of spotted horses in China...or of spotted horses in caves in France (around 18,000 years ago):