The game logic often acts on the first Horse instance it finds, and Horse Whistle uses Utility.getHorse() which works that way. Since the tractor is internally a horse (which is necessary for riding mechanics to work), this can have unintended effects. For example: sometimes the tractor will be warped into the stable instead of the horse, and summoning your horse with Horse Whistle may summon the tractor instead.

Split the tractor into two classes: a generic NPC subclass when the player isn't riding it, and a Horse subclass when riding. That means when horse-related logic runs, it'll correctly find the real horse instead.

Done in develop for the upcoming release.