Instructions
Obtain the lowest honor rank possible. Committing crimes like murder and robbery is the easiest and fastest way to lower your honor rank.
Travel to any general store in the game. Speak with the clerk and sell all of your horse deeds.
Walk outside and press "Up" to whistle for your horse. If your standard horse responds, kill it. This is the only way to break its allegiance.
Whistle again immediately after killing your horse. The Dark Horse will answer your whistle and become your default steed.
Maintain a negative honor score to retain the Dark Horse. Your horse will stop responding to you if you attain the neutral or positive honor ranks.