Taking a break from my grass cutting I went to check on the critters. To my surprise there Izzy was calmly grazing with her sheep. Puzzled I looked at the fence and it seemed okay. Walking around the fence line I discovered Izzy opened the wire gate (think old school with the wire around the fence post) and let herself into the field.
I fixed the gate and left the horse. She loves her sheep and the her sheep love her. They can stay together.