Ol' Bessy is an animal that Old Man Jenkins owns who appears in the episode "FarmerBob." She is the undersea equivalent of a cow.
She has the same shape as a normal jellyfish, but she has a big shell on top of her body with a cow-like pattern on it. She has an udder under her shell and pink legs which likely represent the stingers of a normal jellyfish.
Role in episode
Milking Ol' Bessy is one of the activities Old Man Jenkins tasks SpongeBob and Patrick to do when they are inside his barn. However, when SpongeBob tries to milk her, her milk does not get squirted into the bucket SpongeBob was using to get the milk, but instead, it was squirting it elsewhere. This makes SpongeBob mad and grab all of Ol' Bessy's udders at once and squirt it, but that ends up making Bessy mad and start kicking everything and squirt a massive pile of jelly, which causes Old Man Jenkins, SpongeBob, and Patrick to hit the door of the cabin.
Despite first being mentioned in "Friend or Foe," it took her 11 years to appear.