The Rump People (or Butt Heads) make an appearance in the episode "Freak City." They are thrown into panic when their village was caught on fire, possibly due to Gork's lava breathing. Finn is persuaded by Jake to help the village but when he arrived, he realized he hasn't learned how to use his new foot body and falls on a small flame. A few villagers see him and label him a freak. They then proceed to throw him under the bridge with the rest of the "Freaks." They are not seen for the remainder of the episode, except for the babies that Jake saves. On Pen Ward's Formspring, someone asked what the Freak City villagers were supposed to be, Pen replied: "Butt Heads."
Their heads are shaped like buttocks, hence their name, Rump People. The Rump People are all colored a light blue, and their Rump Babies look just like them, but smaller with pink cheeks. The Rump People wear shirts with multiple stripes across their chests with one shade of color and the stripe across their chest being in a lighter shade.
If a hero going to help you, do not call them freak