The Half Monster Demon is a character who appears in "Return to the Nightosphere." After Finn and Jake ask him how they can talk to Hunson Abadeer, he brings them over to the line that leads to The Teller, who can help them. His insides are "habitable," so anyone who uses him to transport somewhere can survive without being digested. Three Half Monster Demons are seen as Finn and Jake observe the Nightosphere after getting out of jail.
In Adventure Time: The Enchiridion & Marcy's Super Secret Scrapbook, the creature is referred to by name as Distryllag.
The Half Monster Demon is a huge creature that looks like a pink oval cut in half, exposing all of his organs. He is quadruped with long and thin legs that bend very unusually; the front legs have knees that allow for a nearly 180 degree bend and the rear legs have two joints. At the end of these limbs the demon has two digits. He has one big green eye with a pink and purple iris and his tongue is always hanging out of his mouth.
The Half Monster Demon can levitate if he needs to move fast, though he also moves faster than smaller creatures by walking on his long legs. The demon can also communicate telepathically rather than speaking aloud, though his telepathic voice appears to have a minor speech impediment. He closes his eye when communicating.
The Teller can help you. Get in my flippin' belly and I will transport you. My insides are habitable. You will not be digested.