Working for a two vet office here in Wyoming, they rotate their call nights so someone is on call 24/7. However, I realize in some areas the after hour emergency vets are not the normal one on call. Was this vet one of the normal vets from the office?
I doubt you can sue for something that wasn't done in the beginning and then cost more later on. Because I will assume the vet that did not take the call will say" the client did not inform me of a pre exhisting issue of a retained placenta when she placed the call and did not say it was an emergency. I felt the check up could wait until non emergency hours in the morning and I was never informed she called again that morning for a checkup".
If you did not inform the on call vet that the mare did have issues, then the vet assumed it was a normal birth and all was okay. Now, sounds as if the receptionist decided on her own that the call was not important the next morning, and sounds as if she made the decision to not tell the vet you called.
My advice is this: go into the veterinary clinic and ask to speak to the main vet. Or make an appointment to talk to him. Then explain the situation, but be honest with everything you said, and be sure to say you did not explain about the previous retained placenta, but still assumed the on call vet would come out as you asked.(after all they get paid emergency prices for after hour calls and I am sure the main practice owner would like the income)Then explain you called the next morning and the receptionist(name) did not feel it was necessary for the vet to come out right then and although you waited all day, a vet never came. (which tells me the receptionist never gave the request to any of the vets)Explain by the time the next morning rolls around the mare is near death and only then does a vet come out.
If you talk to the main owner, I imagine he/she will do a reduction in the bill to make you happy and to keep you a client since it seems the practice failed your mare and you.
If your mare had died, then yes you could have filed a malpractice suit, but again the first issue brought up would be that you did not say the mare had a history of retained placenta when you first called. You made the first mistake by not informing the vet about the placenta, the vet was lazy and did not want to come out during the night, the receptionist failed the next day it seems and then finally they did come out. I bet your main vet will reduce the amount owed. Just be calm and explain how you feel. Good luck