The Slugs

The Slugs are the titular main antagonists of the horror film of the same name. They are a species of highly mutated black slugs that have developed carnivorous appetites that include human beings - they even have tiny mouths loaded with sharp fangs that are capable of a powerful bite.

The Slugs have a swarming instinct and would mob their victims en mass, devouring them alive - they began to take over a sleepy town and infested the sewers in such great numbers that the heroes of the story had to set the sewer on fire: causing a massive explosion that killed most of the Slugs.