The gelatinous cube is a common monster in all sorts of Role-Playing games, and even in some newer fantasy fiction. The cube is nearly transparent and is usually found in stone corridors. Where they will grow to fill their environment—if a cube lives in a corridor 8 feet wide and 7 feet tall, it will grow to be 8 feet wide and 7 feet tall. They get their nourishment by slowly moving through the corridors of their home, while absorbing everything that they encounter, leaving a trail of slime behind them. They reproduce by leaving bits of themselves behind to grow into full sized cubes. But often they end up absorbing their own young on their next trip down the corridor.
This is not an intelligent monster at all. The cube acts only on instinct, with their only instinct being to eat everything. Because of this they tend to be found in well-traveled corridors where they can feed on a variety of trash, animals, and intrepid adventurers.
From time to time a gelatinous cube will be kept as a guardian or pet of some sort. Powerful wizards or intelligent evil monsters often do this. They are the slime creature of choice for such purposes, simply because they are large and easy to keep track of. Not to mention the fact that they cannot slide underneath doors like so many other slimes and jellies can.