Lady Rainicorn

Nothing to see here, move along. No ideas for stories here. Nope. Not at all.
Ok, so ignoring that, here's my headcanon for Rainicorns, as demonstrated by our lovely little Lady here.
You might notice something very quickly about her; sharp horn, feline pupils, curved and sharp hooves with back claws, and protruding fangs. This is because, implied in the show and explicit in my headcanons, Rainicorns are entirely carnivorous. We see that Rainicorns used to eat humans as a delicacy, as well as that they were believed to have been driven to extinction. This explains the fact that all the surviving direct human descendants are underground, from the Hyoomans to the Super Porp employees, the only places with still functioning human society or traits are all below the ground. So this leads me to the fact that Raincorns are the reason that humans on the surface were driven to extinction by hunting them. So in my headcanon and universe, Rainicorns are pure carnivores.
Rainicorns are magical creatures that are made out of light, and as such, have the ability to become incorporeal and phase through walls or entrap creatures in their bodies by phasing through them. They have the ability to fly by intercepting beams of reflected sunlight and dancing around on them, as well as short range teleportation. This is tied to their incorporeal states, and is one of the reasons Rainicorns were considered extremely fierce predators in the very early centuries after the Great Mushroom War, able to track prey down and even teleport prey into their bodies by using their phasing. Luckily, as Ooo is now a more peaceful (peace being relative here), they have formed a society which forbids eating most sentient creatures (excluding humans, who were assumed to be extinct and therefore not requiring need to be forbidden), and rely on food from other kingdoms to sate their carnivorous appetite. Approximately 70 years ago from Adventure Time's beginning, Rainicorns and Dogs fought for control over the Crystal Dimension, which was a bloody war that ended with a massive victory for the Rainicorns, to the point that dogs still fear Rainicorns to this day.
Lady Rainicorn is in fact, not any form of royalty, despite her name. She was named Lady as a remembrance of the dog who saved her father's life, who had a little pup named Lady waiting for him to come back from the war. Bob and the dog made it safely home from the war, and Lady Rainicorn still keeps pen pals with her dog counterpart. Lady is a very mischievous prankster, she has been in a very intimate relationship with Jake for quite a while, sharing many years of history and several embarrassing or sexual stories. She speaks only in Korean, although knows a very small bit of English that she uses in times of need. She enjoys joking with Jake more than anyone else, laughing together at stories that the two of them share. She is close friends with Princess Bubblegum, who she allows to ride her, a distinction that only Jake and PB are allowed to do, unless there is extreme danger.