I'm sorry this happened to your young duck. You must work now to stop a repeat performsnce: many predators return for more after a successful predation.
Ducks need protecting from predators and, particularly after dusk, that needs more than a fenced back yard. Mine free range in a fenced back yard, but I get them locked in their predator-proof coop before dusk. To be predator proof, the duck house needs to be sturdy -- foxes and raccoons will break into rickety structures -- and the lock must be raccoon proof: raccoons have prehensile hands and are intelligent about unbolting and unlocking doors. The openings in the coop should have half inch hardware cloth coverings -- chickenwire is not strong enough to keep raccoons and fishers out. If the coop does not have a solid integral floor--ie if the walls are stood on the ground--the underneath needs half inch hardware cloth to prevent predators tunneling in. The cloth can be as a skirt burried around the walls although I prefer it completely covering the ground under the coup. My son has had a raccoon try and tunnel under two sides of his coop corners. I saw a raccoon at the end of my road at 5pm in broad daylight yesterday -- I dashed home and seriously wondered about getting my ducks inside at 5pm rather than 7pm. I have made the calculated decision to leave them out after 5pm as the end of the street borders natural preserve. My back garden is surrounded by back gardens all several hundred yards from the preserve. But it is a risk.
Any mammal predator including a dog might have killed your poor duck. We cannot know what did it without knowing the predators in your location
