Nibbles

Nibbles are the Chain Chomp-like fish that inhabit the waters of Rogueport in Paper Mario: The Thousand-Year Door. If Mario falls in almost any body of water, Nibbles will bite him, causing him jump back to same place he fell off, losing one HP. Nibbles cannot be battled.

Grodus had some for pets, in his room. Nibbles are also strangely found in every body of water in the entire game, even fountains and swimming pools.