He is an entity made of green filth. He has a banana peel on top of his head, a sock, a bone, strands of hair, and two leaves are visible on his body. His shape is similar to Patrick's, which makes SpongeBob believe he is Patrick covered in grime.
Role in episode
SpongeBob thinks Filthy Muck is Patrick, so he decides to cover himself in filth as well and name himself "Little Yuck." The Bikini Bottomites form an angry mob against Filthy Muck's stench and want him out of town. SpongeBob defends Filthy Muck, stating that he is his best friend. However, it is revealed that Filthy Muck is not Patrick, but rather an entity made of filth.