Neglected Pony Across the Street - What should I do?

Apologies if this isn't the right spot to post this. I am not a horse owner, nor do I know much about horses and horse health.

I live across the street from a very small horse pasture which contains horses owned by several of my neighbors. They have an agreement with the owner of the land. There are five horses including a pony. The four big horses have shelter and I see them getting fed and ridden from time to time, and I think they're doing okay.

However, the pony has no shelter and stands exposed all winter long, once I saw icicles on his fur! He stands in the same spot all year long, he never gets ridden or has a chance to walk around. I've never seen anyone feed him, but he does seem to get hay on occasion. He is very skinny and appears malnourished, and his hooves are curling a little bit. I feed him apples from my tree from time to time because I feel so sorry for him. I have no idea who owns him and when I talked to one of the other horse owners he said he has no idea either. What should I do? Who should I call? Am I just overreacting?