The King of Rat Town is a mutated sewer rat who lives in the sewer and is the leader of many other sewer rats. He is known both as "Rat King" and "King Rat".
He was once a former lab rat. It is unknown whether he mutated due to experiments when he was a lab rat or was mutated due to living in the sewer, however since many other rats live there not mutated the theory that the Rat King mutated due to experiments may be more likely. He is large, muscular and a common antagonist to the penguins. He speaks with a New Jersey accent. He is also prone to filling his sentences with exclamations such as "Dude!" and "Awesome!".
He first appeared in Crown Fools when the penguins had to get King Julien's crown by battling the Rat King. The Rat King made a second appearance in Miracle on Ice where he and the rats were seeking refuge from their flooded sewer, won a game of hockey against the penguins and banished the penguins from their ice until he and the rats were defeated by King Julien in a second round thus allowing the penguins to reclaim their ice.
He made a third appearance in Roger Dodger where he and the rats invaded Rogers' sewer and banished him from it until Roger switched brains with Rico and became tough enough to stand up to the Rat King and reclaim his sewer.
The Rat King appears to finds birds inferior since tends to use the word "bird" as though it was an insult and calls the penguins "birds" instead of specifically "penguins" (except only once) therefore he is hinted to be anti-bird.
He made a fourth appearance in The Lost Treasure of the Golden Squirrel where he and the rats were competing against the gang (penguins, lemurs, Marlene, and Fred) to finding the Lost treasure. He and the rats failed though because the treasure was destroyed by the gang. He and the other rats were presumed dead, but were last seen barely escaping a pit of molten lava. Their demise is strongly debatable and possibly unlikely and they may even return in future episodes since they are recurring characters, and it is very uncommon for recurring characters to meet their ultimate demise.
He makes his fifth appearance in Rat Fink. He believes that Mort is the great one because of the red stripe on his tail. The stripe is just jelly though.
He appears again in Rock-A-Bye Birdie when the Kowalski, Private, and Rico try find one of Kowalski's invention, a ray that returns spoiled food to its younger fresher state. The ray had accidentally returned Skipper to a younger state. King Rat is revealed to have the ray and captures Kowalski, Private, and Rico. Just as he's about to lower them into the sewer's acidic river, Young Skipper comes in and defeats him by farting in his face.
He appears in Nighty Night Ninja. When Leonard the koala defeated him doing karate moves. Later the rat king wanted revenge on Leonard but the koala defeated him again by freaking him out with love and hugs.
He re-appears in Siege the Day where he and the rats tried to steal the fruit along with the hornets.When they get in the rats and hornets fight over the fruit.Afterwards Private is able to shoo the rats and hornets away.
He has a brief cameo in Thumb Drive where he thumb wrestled with Maurice but failed.
He appear again in Night of the Vesuviuses.
Despite his restricted vocabulary the King Rat is certainly intelligent (or at least, street smart), especially in The Lost Treasure of the Golden Squirrel when he decided to follow the penguins to find the treasure. However he is incredibly overconfident and that is likely his biggest weakness. He is greedy and not above playing as dirty as it takes to get what he wants.
Despite this, he willingly gave his throne to Mort in Rat Fink, saying "I'm just a guy whose good at punching stuff, I can't compete with the wisdom of the Great One." Mort being the Great One. This symbolizes that it is through superior strength, rather than intelligence, that the Rat King is in charge.