Glovie, the behavior you describe is called a stereotypic behavior, and it is not a healthy behavior. It normally occurs with animals who have limited enrichment opportunities or who are bored and/or frustrated. Your description of your pony's environment does not mention a stall or structure. It is possible, having been used to a stall environment, that he is anxious without one (and will need one to get out of the weather). If there is a stall available to him, it may be that he is frustrated being alone. Many horses do not do well singly, and need a pasture partner to keep them comfortable.