Duck

This feature's name is a Conjecture, or is NOT known or proven. Its name may change at any time.

The Duck is the fastest and one of the rarest Mobs (3rd to MOOSTAFA and MOOFIE) in MOOMOO.io. When killed, they give

2000 and

150

Technical

One can use the Booster Hat and/or Monkey Tail to catch the Duck.
Instead of chasing the very fast Duck, try using a ranged weapon to shoot it.
Most of the time, you should just leave a Duck alone until you can corner them. Make sure to follow the Duck so it doesn't get away. Also, in the River, try to get it going against the current and equip the Flipper Hat and hit it upstream so it can't run away.
Don’t try to trap it, as traps have no effect whatsoever.
Daggers are ideal for chasing the Duck.
Try and get it stuck against the edge of the map with a Wall. Then finish it off.
Don't chase it into the Desert Biome, as it might run into a Cactus and die.

The Duck sprite was found in the datafiles before it was released in the game.
There is a similar looking animal, the Duck 1, that has also been found in the datafiles.
In the code, it is revealed that it has a coded damage of 8 but isn't hostile so can't use it. This probably is because it was a Copy and Paste of the Wolf's code, and that part was forgotten to be changed.
The Duck is called the Chicken in the game files but, the actual name is the duck being that names that show in the game are "Quack."
Its name is a debated subject, due to it using the "Chicken_1" sprite, and its name being "Quack", suggesting that it is a fake, an alternate definition of quack, besides the animal noise.