Which season of the year would you put a blanket on a horse?

You would put a blanket on a horse in the winter to save him from getting a cold or even dieing.

However, if you do not clip your horse, it is better to leave him unblanketed. To tell if your horse is cold enough to blanket him, touch his ears. If the are cold, he is most likely cold. If they are not, he is fine as is. Also look for abnormal behavior. If he is standing listlessly with his head down, something is wrong, and it may be that he is cold.