Brain in a Jar

The Brain in a Jar is a Master mode-exclusive pet. It has a 25% chance of dropping from the Brain of Cthulhu it summons miniature version of Brain known as Spider Brain.

Trivia

 * The Spider Brain slightly resembles the Brain Suckler, which latches onto human heads.
 * Likewise, this pet and the object used to summon it may be a reference to a scene from the movie "The Iron Giant," in which the main character is watching a horror movie, and a scientist knocks over a brain in a jar. Upon returning to his lab, the brain is no longer on the floor - two seconds later, the brain, now having developed four legs, two eyes and a mouth, leaps onto the scientists face.
 * That being said, it is also possible, but less likely, that this item is intended as a reference to the arachnotron from the DOOM franchise.
 * The item itself may be a reference to the brain in a vat, a scenario in thought experiments which involves placing a disembodied brain in a vat of life-sustaining liquid and connecting it to a supercomputer, allowing the brain to perceive itself to be experiencing reality as if it were in the outside world.