I just had this same issue. My mare was taken during the night while she was sleeping. It had nothing to do with her being pregnant, as she wasn't at the time. There was no warning as the OP said, and the vet took her despite having access to a feeding trough and even prairie grass. This is pretty upsetting as she was the one I started with when the pack came out, and I had trained her quite a bit... Hopefully, others will report this issue, and it will get fixed; I would hate to lose more horses permanently...
Note: I would have reverted to a previous save, but I had mistakenly and stupidly not saved in 4 hours... So, I will just train the other houses I have instead 😞
