Combusto is an elf mentioned in the episode "The Electric Princess".
After Sky Gunderson attacks Dreamland and causes Elf Alley to catch fire, Elfo can be seen cleaning up some ashes on the floor of a house. Another Elf tells Elfo that they will need a new tenant now that Combusto is gone. When Elfo asks who this Combusto is, the other Elf says Elfo is sweeping him up right now.