The Sleeping Fire Giants are the ancient guardians of the Fire Kingdom that appear in "The Cooler." They are located within the Fire Kingdom, and their existence is kept secret from all except for the members of the royal line. They were created by Moe, as seen in "The More You Moe".
The Sleeping Fire Giants are large metal statues that resemble meditating figures. They have glowing blue lines along their bodies that grow dark when losing power. The giants appear to be visually distinct from one another in terms of shape and design.
The giants are significant sources of energy that can be used by the Fire Kingdom as backup power in the case of an emergency. They appear to rely on a geothermic control module to operate as they power down when it is removed. In addition to being an energy source, the Sleeping Fire Giants are the kingdom's greatest weapons. Because of this, almost all of the giants were disabled by Princess Bubblegum in "The Cooler" as she claimed that they were a threat to the Candy Kingdom.
Flame Princess, and presumably those of the royal line, believe that the giants were the original creators of the Fire Kingdom. It is hinted that there were more fire giants in the past and only six are introduced in "The Cooler." By the end of the episode, only one of the giants remains after the rest were accidentally destroyed by Flame Princess while she was fighting Princess Bubblegum.
- Their origins and purpose are described in the song "A Kingdom from a Spark."
- They were designed by Moe, the creator of BMO. In the episode "The More You Moe", blueprints of the giants are seen in the background of one of Moe's memories.
- In Moe's memory in "The More You Moe" you can see one of the fire giants being assembled through the window, indicating that he not only designed them, but also built them.