The Brain Beast (or Snake Armed Ruby Brain Beast) first appears in "Tree Trunks." During the episode, the beast attacks Finn, Jake, and Tree Trunks in the Evil Forest. Tree Trunks tries to seduce it with her "charm" and "elephant prowess." Finn saves Tree Trunks by hitting the magic gem weak spot on its back, killing it. When the creature dies, it slowly dissolves into thin air.
It looks like a pink brain, with long snakes coming out of it for arms. On its back is a gigantic ruby; breaking this gem will instantly kill the creature.