What is amounts to is this: if indeed the true story is what the OP has stated, with all the truthful facts, no fabrications, then she has been done a great disservice and should indeed get her foal back.
If there are some discrepencies and fabrications that are being given and the BO is within his rights, then guess a court of law will make a decision.
I don't for a minute believe the "stupid officer" just was doing what he wanted, I don't believe that is what happened. He was following what he could do, obvioiusly the letter he had in his hand gave him the reason for not allowing the foal to be taken that night. If indeed he didn't do what needed to be done, then I would have been filing a complaint against the police officer that night.
The OP will have to go to court to get her foal back.. Sounds like the BO believes the foal is rightfully his. She says its hers. I am not there, so can't say for sure, we are only reading what she is posting. Would be interesting to hear the BO's side of things.