El Chupanibre is a sewer monster who once terrorized the mutants below Planet Express. He is a giant green humanoid lizard with massive yellow teeth, claws, and bloodshot eyes. He was flushed down the colossal toilet to the sub-sewer by Bender in the course of saving Nibbler.
The Planet Express crew came upon the mutant city after Bender, under the control of Leela's emotions via one of the Professor's inventions, flushed himself down the toilet in search of the lost Nibbler. Upon meeting the sewer mutants the crew was told the tale of El Chupanibre in musical form courtesy of the mutant Dwayne.
Even though the song says the El Chupanibre eats alligators, they actually eat crocodiles because alligators cannot be flushed.
He also makes an appearance when Leela and Fry are involved in a race during the episode Teenage Mutant Leela's Hurdles