The Grim Adventures of Billy and Mandy (2001)

Season 7: Episode 12

When Billy, Mandy and Grim once again go to the beach, Grim warns Billy to avoid swimming too far, because the "undertoad", a frog-like creature, will try to drag him into the sea. Billy encounters the undertoad, and he barely makes it back to the beach. With the dangers of the sea averted, the guys go back home, but the undertoad is set on bringing Billy down, that he follows him to his house. The next day, the undertoad becomes a constant annoyance to Billy, even at school. Grim and Mandy capture the undertoad, and set sail to release him at sea, where they meat the giant Mama undertoad.