Fluffy
Fluffy the Anaconda was the main antagonist of the episode of the same name. It was sold at a shady pet store owned by the Red Guy. It then proved his sadistic nature by eating Chicken, his female owner's brother. When it succeeded at this, it convinced everyone that it was mimicking Chicken. It then proceeded to ambush and consume his female owner Cow too just before the episode ended.
Personality
Fluffy fits the very concept of a predator. It is relentless and merciless, attacking and eating Chicken the first chance it got. In addition, Fluffy proves to be sociopathic as it violently crushes the innocent yet dim-witted Cow for hugging it.
While its preferred food is chicken but when ultimately pushed to the brink like Cow denying it from eating her brother, Fluffy turns its wrath onto Cow and devouring her. It later eats Cow's parents, possibly for annoying it.
History
Cow and Chicken first encountered the anaconda at the pet shop, owned by the Red Guy. Chicken stumbled upon to its cage and got eaten in the process. Cow would later discover the anaconda but mistakes the snake actually wanting Cow to be its owner but in reality, it was Chicken crying for help.
After being bought and taken to Cow and Chicken's home, Fluffy made several attempts escaping Cow and pursue Chicken but ultimately fail in the end. After Cow unknowingly denies Fluffy of Chicken, the giant snake attacks and eats Cow in anger causing the former to finally realise her pet's vicious nature.
Fluffy then eats Mom and Dad for annoying it just like their daughter. Chicken confronts Fluffy to save his family but gets devoured, just what Fluffy wanted. The next morning, the Red Guy witness Fluffy in horror. However, its revealed that Chicken manages to knock Fluffy out and Chicken demands to have snake taken back for all the trouble it caused. Fluffy is returned to the Red Guy while Cow, Chicken, Mom and Dad are free from the snake.