Alligator

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.

Alligator is a 1980 horror thriller movie directed by Lewis Teague, starring Robert Forster, Michael V. Gazzo, Dean Jagger. It holds an IMDb rating of 6.1/10 from 18,433 votes and runs 1h 31m.