My horse looses weight in the summer. She gets fed regularly, and has a perfectly fine weight in the winter but suddenly when the summer starts she just looses all of it to the point where her ribs are just barely showing. I usually give her some extra beet pulp or soaked alfalfa cubes in the summer, as well as letting her graze as much as possible so she doesn't loose any more weight, but I think it just kind of happens to some horses.. Some horses are easy keepers, and some are hard. You can try feeding some sort of supplement, but if you're really worried about it then maybe talk to your vet. Also like Dustbunny said, there are a lot of factors that could be taken into account as to why she's losing it.