Strange but neat - turns out a few guys like Curly Bill Brocious and Pony Deihl who we know as the red sash Cowboy Gang in the movie Tombstone (and real life) were in the Jesse Evans gang earlier, where Billy the Kid at one point rode with them. Once the Lincoln County wars broke out, Billy the Kid left and joined the Tunstall Lincoln County Regulators to fight sheriff Brady and the Evans gang.
So, the same guys chasing Billy the Kid and the Regulators in Young Guns were the same dudes who later became the bad guys in Tombstone in the Cowboys gang.