Bat Bugs are flying creatures who make their first appearance in the episode "Evicted!" They cover the walls of Marceline's cave, leading Finn and Jake to mistakenly believe the cave is dirty. When Finn and Jake enter the cave, the Bat Bugs fly away, revealing a clean, glittering cave. They also appear in Level 2 of "Righteous Quest" and in "Fionna Fights!" and are available as an overlay in "MiXiT Video Creator," where they are called "Adventure Time bats."
Bat bugs appear to be bugs with wings similar to bats. They have various shapes and sizes. They have a green skin color and a green eye color.