Loner duck dragged around by the neck by predator

[CrookedFurrows]

I have 3 Pekin ducks and 5 Swedish ducks all the Pekins are girls and I think two Swedish are drakes, as well as 5 chickens (going to get more). The largest and heaviest of my pekin girls is either a loner of just plain stupid because she will hang out in the barn or just stray away from the duck flock, if that weren't enough she advertises her position by loudly quacking. Recently we've started losing some chickens so I've taken to locking the bird up at night AND while im at school. Since then we haven't lost any chickens but the weekend before last I came home to the largest duck with holes in its neck and blood caked to its stomach. I cleaned the wound and put some iodine on it and in four days she was fine but she still wandered away from the flock and this is not the first time shes had holes in her neck (same duck!) last year we had a fox grab her and drag her around everywhere before letting her go in the middle of the garden. Does anyone know how to prevent this further? and maybe why this one duck acts so oddly?