Lethargians

The Lethargians are a group of lazy slimy shape shifting creatures featured in the book and film "The Phantom Tollbooth", they live in The Doldrums a dark swampy place filled with what appears to be mucus, they have rules for living there such as no thinking, no laughing, and smiling is only permitted on alternate Thursdays, they spend most of their time lying around, sleeping, or converting from solids to liquids.

At first glance they appear to be relatively harmless but they are far more sinister, they lull their victims to sleep and eat them, Milo the protagonist encounters them while lazily driving down the street in Dictitonopolis ignoring the warning signs, his car gets stuck in the mucus and they introduce themselves and explain the rules for living in The Doldrums, after some time they lull him to sleep with a song and prepare to eat him, they are stopped by Tock the "watchdog", in the book they flee in terror and aren't heard from again, while in the movie they return after being chased and turn themselves into a tidal wave in an attempt to kill Milo and Tock, Tock convinces Milo to get his brain working which in turn causes his car to start up again, they evade them by swerving near a cliff and they fall from it presumably to their deaths.