I would just about bet that you have a hybrid. Amallard will cross with about any duck it can run down and rape. It may a tame duck of about any breed crossed with a mallard. These are quite common among flocks that have free flying mallards around them. It is highly unusual though for them to leave the safe secure of a barnyard flock and join a wild flock. It does happen though. If I had a digital camera I could take a picture of one like that. It has been here at my place for 2 yrs now.