You may be aware of my ongoing claim that Adventure Time with Finn and Jake is the most D&D cartoon of all time. (Sorry, Daffy. One wizard episode does not a claimant make. And the Dungeons & Dragons cartoon series? Not even close.) But let's go beyond some of the obvious reasons why (alignments, character class names, nine magical spell levels, references to D&D monsters, killing monsters & looting their treasures, ad infinitum) to consider two good and weird pieces of evidence.
One is the fact that the world of ATwF&J is a post-global-holocaust world (references to the Great Mushroom War, countless background scenes containing ruined objects from our world). The other is that an important Lovecraftian motif pops up in the episode, "Susan Strong." Going down into what would appear to be a massive bomb shelter, Finn and Jake discover a lost tribe of humans who wear hoodie-hats that look like different animal heads.
SPOILER ALERT!
.
.
.
.
.
The lost tribe members are not humans, but hyoomans. When they remove their animal-head hoods, they reveal themselves to be mutants. Deep Ones, to be precise: complete with scales, fins, and gills.
At the end, Finn wonders what is underneath Susan's hat.
Finn: What about you, Susan? What are you? Was she a human like me or an animal?
Jake: We're all animals, brother.