I have heard rosehips are good for horse joints. And yes, fresh rosehips are fine. You can freeze them to save them for the rest of winter! 1-2 handfuls is a good amount for fresh rosehips, but 15-20g of dried. The amount should be halved for ponies. Feeding to much can cause diarrhea, because of to much ascorbic acid content. Apparently, another option is to plant them in your horse's pasture, so they can choose how much they can eat, and it sounds like Sam did that!
“Good things come to those who wait… greater things come to those who get off their ass and do anything to make it happen.” - Unknown