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 In a world with The Crimson.

Trivia

 * 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 - 2 seconds later the now spider-brain leaps onto the scientists face. The spider-brain from this scene has two eyes, 4 legs and a mouth - similar to an arachnotron from the DOOM franchise.
 * That being said, it is also possible, but less likely, that this item is intended as a reference to DOOM.
 * Or, y'know, maybe it originated from outside of geek culture, like with the well-known thought experiment of the same name.