Sparkling Honey

Master Mode is the gate. Beating the Queen on any lower difficulty will never produce this, so if you want the pet you either play the whole world on Master or set up a separate one just to farm her.

That is less painful than it sounds. She is one of the cheapest bosses in the game to re-summon and her arena is a hive you can prepare once and reuse, so repeat attempts cost little more than travel time.

The pet itself is a fat bee that trails behind you and does nothing at all. It goes in the dedicated pet slot, so keeping it out costs you no gear. The pot it comes in is a nod to a certain honey-obsessed cartoon bear.