The Sewer Gator is a huge alligator that dwells in Dimmsdale's underground sewers.
He was the subject of a very violent documentary that Timmy Turner wanted to watch, but was not allowed to by his overly-concerned and protective parents due to Timmy being rough with playing with a soccer ball. As a result, he got laughed at by the other students of Dimmsdale Elementary School. When Timmy wished that his parents didn't care about anything anymore, he was allowed to watch the documentary, but his home life soon fell into ruin as his parents neglected hygiene, didn't pay the bills, and forgot to complete household chores.
Timmy finally found a way to return them to their previous mindset by running away from home, performing various dangerous stunts, and finally encountering the Sewer Gator for real (Timmy even noted that his parents were going to kill him for all that, but notes it is the only way to get them back to normal). The Sewer Gator nearly killed Timmy, but Mr. Turner attempted to brave the alligator himself, in order to save Timmy and succeeded. Due to Timmy doing a lot of things that were dangerous and being rebellious towards his parents, he was grounded for a couple of months as punishment for his actions.