We recently had something so similar happen with our old girl (21). Sissy has had a bad hip for years and so we brought her into the barn this winter and she was really doing well, weight was great, coat was thick and shiny, etc. We did hooves about a month ago and she HATES having her hooves and it was a bit of wrestling match to get them done. Following that she seemed a bit depressed and perhaps in a bit of pain, so we gave her some banamine for a couple days but she just slowly went off her feed and wouldn't eat. Wasn't colic because she was pooping just fine while still eating. We started giving her gastrogard thinking perhaps it was an ulcer. Her breath was awful and she started licking the ground alot, particularly where we had put salt down to melt some ice. When she developed ulcers on her mouth we got the vet out and they did blood work etc. Kidneys were OK but something appeared to be wrong with her liver (whatever that means. . . our vet isn't terribly up on minis). Could this be hyperlipidemia?
Anyway, the vet had us give her a shot of some antibiotic and continue with the gastrogard. She seemed a little bit better but still wasnt eating very much. Nose at her hay and she'd drink the "slurry" from us putting water in with her senior food, enough so that the diarreha (sp?) turned green but she just wouldn't snap out of it. Last saturday we couldn't even get her to sit up. We left her stall door open and mid-afternoon, she got up, stumbled out of the barn down to the pasture with the girls she'd spent her life with, whinnied at them one time and laid down and died.
Still don't know what the original problem was and though we wanted to get her through this winter, we were seriously considering not letting her go through another MN winter. Didn't want lose her yet--she was our first mini--but I guess it was her time. I wish I understood hyperlipidemia better . . .
Anyway, sorry to hear about what you going through with your mare. . . It's always hard to say goodbye but I, at least, was comforted by the knowledge that Sissy had a great life. . .