We had something similar happen, just not with a horse, but with a cow. Our neighbors bought a little heifer (about 10 years ago) and didn't have any place to put her, so they asked if they could put her in our pasture with our cows and they would pay any expenses. Well, some time later, the little heifer got real sick and the owners had no interest in paying vet bills, so they said that we could have her if we took care of the bills. Now, jump to last summer. We had her bred and she had a gorgeous calf, but could not nurse, so I started bottle feeding. For some reason, the neighbors wife wanted to bottle feed the calf, but my kids wanted to do it. So, she started telling everyone that the calf, cow and another of her babies from three years ago were theirs, not ours and that they wanted them back. So, I wrote up an itemized list of expenses and presented it to her. I didn't ask her to pay, but just told her that THIS is what she owed. And, I said that I was feeling generous and didn't add the vet bills that started this whole thing. Of course, the cost was outrageous. And, the husband told her that they were not going to pay that just so that she could have a little calf to bottle feed! We haven't heard anything else since.