The bug

When being ridden for the first time, horses have an internal counter that is not saved with the game that tracks how many times a horse has stepped while being ridden. If it is above a threshold, the horse makes galloping sounds instead of "wood" sounds. However, this counter is never reset unless the world is reloaded, even if the horse is dismounted. This causes the horse to make gallop sounds even when the horse is walking around unsaddled being ridden by a player.

To reproduce

Spawn a horse and begin riding it. Wait until it start produces galloping sounds while walking. If the horse stops and starts again, it will continue making galloping sounds (even when going at slow speeds). Dismount and remount the horse, and it will still make gallop sounds.