The Brain Follower is a pet brain released from the Brain in a Bottle. It drifts around the player, doing quick movements. As it resembles the Brain of Cthulhu, it can phase in and out from the visible eye. It will do this about every six seconds.
The Brain in a Bottle is crafted from 20 Vertebrae, 10 Bottles, and 60 Tissue Samples at a Workbench.