Otyugh
Otyughs are creatures that literally eat shit and rotting meat, and have been around since 1st edition.. They tend to live in gross places where there is plenty of food, like sewers and garbage heaps. Otyughs are so infested with disease and rot that they'd probably make papa Nurgle proud, and sometimes they are used as living garbage disposals.
Amusingly, in their natural state, otyughs are actually quite docile and benevolent creatures. It's just that the ones you encounter in the average dungeon crawl are usually half-crazy from maltreatment and inadequate nutrition, or have been actively trained by the dungeon's denizens to act as an attack dog.