Throw the mare outside and hand raise the foal. The foal can still be saved.
Stomp the foal in front of her and break her spine. It’s the only humane thing to do.
Cover the foal in that bitter stuff that’s used to make kids stop biting their fingernails. Then wait for the bitch to lick the foal clean.
The mare looks semi-derped already, so finish derping her, take the foal and raise it or give it to another mare, keep the original as a source of comedy.