The Crater Critters are large, brutish rock creatures who bathe in lava and claim to enjoy "eating" brains. They only appeared in the episode Last Action Zero. They are voiced by Jim Wise and Dee Bradley Baker.
At the beginning of the episode, a critter is seen relaxing in a volcano where he is heard talking about wanting to eat brains. Jenny and Brad interrupt his vacation and thus leading him to get into a fight with Jenny. Brad tries to contribute to the fight by getting ahold of Jenny's Net-o-Matic but screws up as the critter traps the two. The critter attempts to crack open Brad's skull but is frozen by a 7067 Skyhawk along with the crater being frozen as well. He is arrested by the Skyway Patrol and taken to the ship. After Brad is recruited, a ship filled with critters arrive at the crater and thus blame Jenny for freezing it and attack her. Long after, Jenny is badly injured and continues to fight, but the critters are immortal and thus attempt to eat her brains. The Skyway Patrol comes to the rescue to save her, but Brad is incapable of doing so due to the paperwork he had to sign. The critter that was arrested at the beginning of the episode breaks free due to the deportation forms being awaited. Jenny's head is cut open as the critters try to eat her brains, but they are immediately frozen by the Skyway Patrol and Jenny is rescued. Jenny and Brad battle the critter by shooting paper at it because rock is weak against paper (according to the Rock-Paper-Scissors game).