Jack Horne's wife and child were Native Americans.
According to the Pigeon Brothers, Jack made a name for himself by killing Crow. Historically, the Crow Tribe were allies of the US government and worked with them against their enemy tribes such as the Sioux and Cheyenne. Perhaps Jack's wife was from one of those tribes.This would explain why Jack ended up with a bounty on his head; he killed Natives who were working for the government and passed their scalps off as scalps from a different tribe. It would also explain why he shows no prejudice against Red Harvest. He doesn't hate Native Americans in general, just a specific tribe.
Billy Rocks was originally a railroad worker.
A large number of Asians in America during this part of history were laborers recruited to construct the railroads. He also shows some expertise with dynamite, which he would've likely obtained if he were involved in blasting the countless tunnels the Central Pacific had to carve through the Sierra Nevadas. Goodnight says he met Billy while serving a warrant from the Union Pacific, implying that Billy and the railroad had a history together and didn't part on good terms. Joshua Faraday is an ancestor of Peter Quill
Someone had to suggest it.