Although your stallion got out and inseminated a mare, she must be cycling with a viable follicle in order to be pregnant. If you do think that she was cycling and had a viable follicle, you can ask your veterinarian to examine her. He will probably need to wait at least 14 days, then ultrasound her and perform an exam to be sure that she is pregnant. There are a number of methods that will terminate pregnancy. It is possible that the attending veterinarian will utilize an injection of prostaglandin F.

Also, double check your stalls and fences and make sure that your stallion will not be able to get out again; or if you don't plan on breeding him, consider castration.