If you have a horse item that was not yet transferred to a horse, and you purchase another similar item, it will no longer expand its life, you will have 2 similar items.
This way if you want to buy similar 2 items for 2 different horses, you will no longer have to transfer the first item before you can buy the second.

On the horses' page there is also a button that will help you get a magic item for the horse. The game will only show the possible items based on the horse's age, gender, status.