My girl's not near as thin as she was say... a week or two ago... and personally, I think she looks pretty good, though I'll be the first to admit she does need to put on a few more pounds in both weight and muscle. I haven't been riding her hardly at all (I think I've only ridden her once since the second week of April... and that ride was for maybe... twenty minutes...). She isn't getting anything grain-wise except for when we ride (thus, she currently isn't getting any grain... I don't have the money to feed her consistently through summer when there's plenty of green grass to eat...). She's on 24/7 pasture grass.
I'm not too worried over her weight, though, because she gets thin at the beginning of every summer (and has been doing so since '06). Her teeth currently do not need floated and she is up to date on deworming. She's on the 'getting older' side at 15 years last month. She is healthy, just thin.