To get her hind end more thick and rounded and less drop offy, I would lunge her standing on a hill side. This gets the hind end working more to pull themselves up and down the hill. Just do that for about 10 minutes both ways maybe 3 times a week and you will definitely see an improvement. Also, she looks pretty old from the picture, so you might want to check if she needs her teeth floated. Does she drop a lot of food when she eats? My horse was as skinny as a twig and I tried everything to put weight on her (she was a rescue horse) and when I got her teeth floated she filled out SO nicely. She is also pushing 19 years old. Also, if she doesn't get too hot from eating certain foods, you might want to try slowly easing her onto some beet pulp if she doesn't already get it with her feed. This really helps pack on the pounds without the extra AHHHH I GOTTA RUNNNN I HAVE TO RUN OVER YOU AND KILL YOUUU!!! Kind of attitude hahaha.