Giant slime

Giant slimes are large, green oozes that prowl in dark, wet areas like sewers and swamps to find prey.

They usually subsist off of fungi and plants, but when a more substantial meal wanders past, they pounce and slay them, dissolving all organic materials before moving on to find other prey.

As such, they sometimes contain coins and other valuables from previous victims. Their powerful enzymes can break down almost any organic substance, but stone, metal, and other similar substances are completely untouched.

The description of various "genera" and "species" of corrosive mineral oozes encountered by unwary miners underground, that can eat unnoticed through skin and flesh to the bone, date back at least to the works of Georg Agricola