I don't see anything glaringly wrong except that she is severely under weight. She may have a SLIGHT roaches back, but I think this has more to do with her condition than her build. I think in better condition she actually could be quite nice. If she is your please feed her more! If not yours, I would love to see her in a home that will take care of her basic nutritional needs! I would not suggest riding this mare until she is in better condition.
She is not really ridden or worked at all and she's on double normal rations plus senior feed (as the vet suggested) and has hay in front of her hay 24/7 and is often lead to other grassy areas for extra grazing. When they got her she was almost twice as thin as this (I don't have any pictures of her from then but she really has put on quite a bit of weight)
She was also pretty spindly when they got her and feeling her now, I've noticed her legs and hind quarters and neck have become thicker with muscle just from her bossing everyone else in the pasture around.
She is well cared for, just waiting for time and her feed to work on her at this point.
Her hindquarters are very odd. The croup is seriously steep, almost as if she's tucking her tail between her legs. The femur is also very steep and short. She looks like she might be the tiniest bit behind at the knee. Her should looks to be large and maybe powerful, eventually.
^ agree steep in the hip but looks worse from no topline muscle-- fed up shell make a good what ever you want horse- her conformation aint bad at all.. its good- just needs more muscle and fat.. she's got a nice head too- most twhs have that long ol mule head, lol.
I want to see her when you get her fed up right and rode a little bit to build up her muscle!