Ducks

Ducks are background critters. As all other critters, they only have 5 health, no defense, and is often killed instantly by other mobs or the player. Their counterpart is the Mallard Duck.

Ducks can usually only spawn in the middle third of the above-ground world, or in areas near an NPC house if water is present. They only appear in pools of water, despite their size or if they are naturally generated. They quite frequently make a "quack" sound when nearby.

Ducks can be caught using a Bug Net. They can be carried around in the inventory until placed (released), deleted, or crafted into a Duck Cage. Caged ducks can be used as furniture and cannot be released once crafted. They also move around inside the cage.

Trivia

 * Very rarely, when ducks quack, the sound will be a very obvious human voice saying the word "quack." This was included purposefully as an "easter egg". It is suggested that this could be Redigit's voice, but is yet not confirmed.
 * They are very similar to bunnies, as they both spawn in the center third of the map or near NPC houses, they both have an alternate version (Squirrels and Mallard Ducks), and being critters, they all share stats.