The Grumper is a large and monstrous frog that resides in Pussywillow Hollow and always tries to eat anything that's smaller than it. According to Sassette Smurfling, it is the biggest frog in the world.
In "The Pussywillow Pixies", the Grumper tried to eat Clumsy Smurf and Pansy, but they managed to avoid it.
In "The Most Unsmurfy Game", the Grumper attempted to eat the Wartmongers and almost succeeded in eating King Bullrush.
In "Reckless Smurfs", Sassette went to capture the Grumper after she was affected by Gargamel's Daredevil Dust. Surprisingly she was able to not only catch the Grumper but tame it as well. However, Papa Smurf thought it'd be best to return it back to its natural habitat.