Alligator (1980)

It lives 50 feet beneath the city. It's 36 feet long. It weighs 2,000 pounds...And it's about to break out!

After being flushed down the toilet, a normal alligator turns into an unstoppable monster. Animals used in experiments are a steady diet for the creature. Soon it comes to the surface and begins to make the citizens in the community it's next meal.