This drake, left (with tail curl from Mallard ancestry), and duck, centre, are "Crested" Ducks. The "bobble" is usually the result of crossing a bird with a crested gene and one without. Only about 30% of the resulting offspring will sprout crests. If two crested ducks breed, the offspring will usually die. As domestic ducks they are usually bred for ornamental purposes.
Crested duck with Call duck
An accidental hairnet
Appleyard and Crested ancestry
A very mixed up hybrid (wild/domestic/crested?)