We eventually decided to tie them out in the yard and wait until morning to try to find out where they came from. The girls were thrilled beyond belief to have foster ponies for the morning and were busy finding clover and grasses for them to eat.
To show how little we know about horses, we initially thought they were mama and baby. After talking with the owners we discovered that they are both pregnant females and the little one was 10 years old while the bigger one was 12!
Just another randomly interesting day out in the Boonies!