Sheldon was a turtle that Stewie Griffin brought home as a pet, but was a homicidal mainiac that tried to kill him and was a one-shot villain on Family Guy.
He appeared in "Lois Comes Out of Her Shell"; Stewie finds him in a pond in the park and decides to bring him home as a pet. What Stewie did not realize that Sheldon was a sociopath that wanted to kill him. It is unknown why he wanted to, but one reason is that he did not want to be taken from his home. He would pull sick sadistic jokes on both Stewie and Brian, making one mad at the other until Stewie realized that it was Sheldon the whole time.
Sheldon first ate Rupert's left eye and then tried to kill Stewie in his sleep. Stewie flushed Sheldon down the toilet and thought he was rid of him forever but Sheldon survived and slowly made his way back to Stewie, but not before strangling a few seagulls that tried to eat him. Sheldon then sent two packages to Stewie as a sign that he's coming for him: Rupert's ripped head and the head of an unknown guy.Stewie prepared himself and lured Sheldon into a trap. The two have a knife fight throughout the whole house and Stewie gains the upper hand and cracks open his shell. The shell was a decoy with a bomb and it destroys the living room and knocks Stewie unconcious. Sheldon then prepares to stab Stewie but out of nowhere, Super Mario appeared and stomped on him; paroding the Super Mario games. It is unknown if Mario killed him or just knocked him out.