Just like their hats, witches’ cauldrons are exposed to an abundance of magic, moreso in fact. Witches use their cauldrons for cooking their food and creating potions, and while they are meticulous about cleaning them between uses, magical materials inevitably leave some residue. With the right combination, something awakens in the iron of the pot, something living and hungry. This is easier in the far north, with its long winter nights.
If a witch realizes her cauldron has come to life, she will immediately discard it and seek another. This is for the simple reason that a pocus is rather useless to a witch. Any food placed in it will be devoured by the pocus itself, and it will even grow mobile feet to seek out more food itself (these feet can be grown from the same metal as the pot, or, disturbingly, organic in nature).
A pocus will eat anything, and it is surprisingly capable at hunting. A pocus’ body is still as hard as whatever material it was made from (iron and bronze are most common, but rarely a witch might have something more expensive come to life, much to her annoyance) but magically malleable, capable of bending to form appendages or hide. The pocus is slow, but it is an effective ambush predator, often hiding in shallow water to catch fish using it as shelter. When threatened, they can spray boiling digestive fluids.
While as mentioned, a pocus will eat pretty much anything it can get its mouth around, its preferred diet is magical ingredients. Anything a witch would use in her potions is greedily guzzled by any pocus who finds it. Newts, glowworms, winterberries, all are scooped up and dumped over the pocus’ rim. Their favorites, however, are witches themselves, a distant second being hocus. Very few will ever get the chance, given witches hate and avoid them, but if they ever do manage to devour a witch, they supposedly become significantly more powerful and intelligent.
These pocus are shrouded in legend due to their immense rarity, but according to some witches they are quite sought after by the most deviant of their kind, as their stews can brew potions unlike any made by a regular cauldron. Just last year, a forest witch in my city was arrested for murdering another witch and attempting to feed her to a pocus in order to create one of these creatures. I have to admit, I almost wish I could have seen the result. What a beast that must be!