Futurama -- Episode 14 -- I Second That Emotion
November 21, 1999

Fry: So, is it true that alligators flushed down the toilet survive down here?
Green Mutant: No, that's just an urban legend.
Bender: Then what are those?
Green Mutant: Crocodiles.
Three-armed Mutant: We keep them as pets. Then, when they grow too large, we flush them down into the subsewer.

(Thanks to Sewerfriend Heather Gilmer for this item.)