A monster created due to Gary Gygax' laziness and the cheapness of 6-sided dice compared to actual miniatures, the Gelatinous Cube is perfectly adapted to a life in an environment consisting primarily of 10 foot by 10 foot corridors.
You might think that such a ridiculous creature could never be a threat, but it is deceptively stealthy and fast.
Once engulfed by the beast, its meal is quickly dissolved until only bone, stone and metal is left behind.
Of course, gold is metal too, and gems are stone. The Cube is often hunted by adventurers hoping to retrieve such indigestible waste, if only for the creature's own comfort.