Alligator (1980)

Just when you thought it was safe to go back into the sewer with Robert Forster, turns out it isn't.  This classic 1980 creature feature follows the titular swamp dweller finding a new home in the city sewers of America. When a young girl buys a baby Alligator and her dad is upset and flushes it down the toilet, 12 years later that tiny, adorable beast has grown into a massive 20 foot long gator who is eating house pets and water and sewage workers on a daily basis.  When detective David Madison (Robert Forster) goes down to find the other half of a body that turns up in a water treatment plant, he's face to face with the beast, that promptly eats his partner.  Now David is on the hunt for a real-life monster who has a taste for people!