Little is known about the background of the Rainicorns, other than there was a Rainicorn-Dog War. One dog saved Bob during the war, so he and his family Ethel and Lady are friendly to dogs. Most Rainicorns speak Korean, though not all of them do.
Rainicorns are unicorn creatures with almost a full rainbow of colors down their body. They range in length; Ethel is about twice as long as Lady, who is in turn three times as long as Bob. Bob is roughly the length of a normal horse, though shorter in height.
All Rainicorns possess the ability to fly by intercepting beams of reflected sunlight and "dancing around on them." They also have the ability to change the color of any object by shooting it with a chromatic beam fired from their horns, and the power to stretch their bodies out to various lengths, much like Jake. They also age much faster than other creatures, as after a little more than two days Jake and Lady Rainicorn's children were already physically older than their father though due to them being Rainicorn-dog hybrids, true Rainicorns might age faster than two days.
- Charlie (Rainicorn-Dog-Shape-Shifter Hybrid)
- Kim Kil Whan (Rainicorn-Dog-Shape-Shifter Hybrid)
- Jake Jr. (Rainicorn-Dog-Shape-Shifter Hybrid)
- T.V. (Rainicorn-Dog-Shape-Shifter Hybrid)
- Viola (Rainicorn-Dog-Shape-Shifter Hybrid)
- Bronwyn (Rainicorn-Dog-Shape-Shifter-Bear Hybrid)
- Charlie's Son (Rainicorn-Dog-Shape-Shifter-Unknown Species Hybrid)
- The term "Rainicorn" is a portmanteau of "rainbow" and "unicorn."
- Jake claimed that for thousands of years the Rainicorns battled the dogs over territory in the Crystal Dimension. The outcome of the war is never outright stated, but it can be inferred that the Rainicorns have some stake in the dimension today as Lady Rainicorn's parents come from there in "Her Parents."
- In the "Stakes," mini-series episode Everything Stays , Two Bread Tom lists one of the human's fears are "those hungry looking rainbows", obviously referring to the rainicorns.
- Rainicorns eat humans, as in the episode "Her Parents," Ethel and Bob Rainicorn try to eat Finn. After the seeming extinction of humans in Ooo, they moved to a soy-based substitute. After Jake prevents this they settle for a picnic of "soy people," which is supposed to taste just like the real thing. Finn's reaction to tasting it implies that even humans like himself find it delicious. Rainicorns eating humans is a possible reason as to why there are no humans on Ooo apart from Finn.
- Rainicorn is 무지개콘 (pronounced 'mu-ji-gae-kon') in Korean.
- Even though Rainicorns are rainbow hybrids, they do not have the color orange on their bodies.
- The episode "Jake the Dad" showed that Rainicorns age rapidly, as Lady and Jake's pups developed quickly from infants into young adults over the approximately two days when Jake was visiting, and once he returned to live in the tree-house with Finn shortly after this, he said that they were already effectively older than him.
- Seeing how Lady hasn't seemed to continue to physically age during the show, (She grew to adulthood within a week yet still looks physically the same for five years) it seems the rapid aging slows down at some point, or Rainicorns despite the aging can still look young for a long time.