Watch a great movie called Alligator.
Alligator 1980
A pet baby alligator is flushed down a toilet and survives in the city sewers. Twelve years later, it grows to an enormous size thanks to a diet of discarded laboratory dogs injected with growth hormones. Now, humans have entered the menu.
Robert Forster * Robin Riker * Michael V. Gazzo
https://www.youtube.com/watch?v=XhmskrE-oGw