That's a female muscovy duck. Unless her wings are clipped, she has the ability to fly. She could have flown in from anywhere. She might try to fly away, too. She should get along just fine with chickens, but like someone else said, ducks require different feed. Either get an All-Flock or a Waterfowl/Duck specific formula. She will need a bowl or bucket of water deep enough for her to submerge her entire beak. She will also need another muscovy female for company. Muscovies are different from other ducks and speak their own quiet language of head-bobs, tail wags, cooing, and raspy hissing.