How much does a shetland pony eat?

I'd get some kind of complete feed and give her just 1 cup per day plus a couple of flakes of hay and that's it. Shetlands are notoriously piggy about eating. If the hay is good quality, she'll eat it if she's really hungry. It's going to want grain over just hay but it's really dangerous for her to gain a lot of weight because of the risk of laminitis. If you are giving it the feed in the morning and feel bad about it only getting hay at night, give it a chopped carrot and apple with the hay and that's it.