@BirdNut: Thanks, BirdNut. It never gets easier. Her trouble breathing and gasping were pretty much the only symptoms, and those seemed to be chronic. When she got really sick the first time I put her on a dewormer and then a broad-spectrum antibiotic. Eventually her crop stopped moving and I had to try and move it for her--whether or not my massaging worked, eventually it started on its own again. She was fine for two weeks then the symptoms came back, and she didn't respond to the antibiotics this time. I had a stronger antibiotic on hand, but if the broad-spectrum wasn't doing anything, I figured it would only make whatever bacteria were inside of her more resilient.
@pinkwindsong: I'm sorry to hear about your donkey and mares
It's sad about the grey horses, I'd heard they get tumors easily. Melanomas, I think? I agree with you on the cancer issue--humans do get more attention than animals on the subject. I think I heard that ovarian cancer, even for human purposes, is often studied in older hens. I try hard to feed my chickens well, but sometimes you just can't delete all the factors.
@nurse_turtle: Thanks for the input on the blood clot! I thought it was normal in a recently deceased animal, but I wasn't sure.
@Elphaba2140: I hadn't thought about it that way, it's very possible! I'm not sure what would cause her heart to enlarge so much--besides the clot and the size, it seemed healthy.
@darkmatter: Thanks for the input. There was a suggestion of eating her but I put that idea down really quickly--looks like it's a good thing I did! That would be cool to see, the quartz and granite in the gizzard. Apparently we just have plain ol' boring rocks here.