Othyugh,
Sentient subterranean beings can coexist with otyughs, employing them as garbage disposals. With such plentiful sustenance, otyughs grow fat in their wallows, unmoved by any other drive or desire. This sedentary gluttony makes them reliable guardians. As long as it is fed, an otyugh refrains from attacking other creatures. However, would-be otyugh masters can easily underestimate the quantity of waste, carrion, and meat necessary to keep an otyugh from wandering off in search of food. More than one “trained” otyugh has eaten its keeper after devouring all the waste in its wallow.
Some of the core things of Othyugh that make them fun include:
Limited Telepathy. The otyugh can magically transmit simple messages and images to any creature within 120 feet of it that can understand a language. This form of telepathy doesn't allow the receiving creature to telepathically respond.
Othyugh are one of my favorite monsters ever, I had this TSR toy as a kid and it played so many roles in out toy stories!
Armored Catfish, not really related to catfish, they live in large schools on the beds of rather filthy water. They don’t pose a dangerous threat on their own, but because they are armored and lay on the floor, they can cause people walking through tainted waters to loose their balance or trip, they are also large enough that occasionally they might run into someone’s feet and cause them to lose their balance, plummeting them into the tainted waters.