Ducks are optional animals that can be obtained by owning a pond and waking up after your significant other in the Summer. You will be given the option to adopt the two ducks in your pond. After being named they will be placed in the chicken coop. You must have no more than 6 chickens to make room for the ducks. If you choose not to take the ducks the option will be lost forever.
Interestingly, ducks do not lay eggs and they instead appear under chickens. There's no labelled difference between chicken and duck eggs, so hatching chickens or ducks is random. Ducklings sell for 300G and ducks sell for 500G.