When you say Horse heavan you mean a muddy slippy wet mess right. It is fine most of the year right now if has been very wet so the fields look like it is the end of winter already so the ones living of the hill are having to have hard food as well as hay which is the first year they have need something more, though it is the first year we have had two in foal mares up there and a yearling. Our Shetland stallion gets rugged no where near as much as this summer and last winter as he spent 6 out of the last 13 months in a stable due to different illnesses and injuries. I would spoil them all and rug them all but because they don't need it mum won't buy them rugs, not that we could really afford to buy all 12 of them multiple rugs and it would be really hard to find rugs to fit the Shetland as just finding headcollors that fit them is next to impossible.
Rugging a horse needs only to be done in fine coated horses in very cold weather (again - IMHO)