Horses appear to be rare as the game progresses, not respawning. This makes taming them more difficult than needed. There are a lot of horses left standing around when players disappear. Having a taming decay act on horses that are not inside a claim would put the horses tamed and abandoned back in play. This would also allow a player to abandon a horse on purpose when they improved their mount, and would ensure that horses were brought in from outside the boundaries of the claim when not being ridden.