Merpeople

Merpeople were briefly mentioned in "Dipper vs. Manliness" by the Manotaurs, and one named "Mermando" appears in "The Deep End." They are known to live in water. It can be inferred that they are largely the same as mythological merpeople and are some combination of half-man, half-fish.

History
What may be a skeleton of one can be seen in the Mystery Shack gift shop, although it resembles the famous hoax Fiji Mermaid. In the opening credits, a picture of a merperson like the one in the Mystery Shack can be seen at the end of the credits. They also appear in the episode "The Deep End" as Mabel befriends a beautiful one named Mermando. According to Mermando, Merpeople have about 17 hearts.

Trivia

 * The manotaurs do not seem to like the merpeople. At one point in Dipper vs. Manliness, Chutzpar mocked them, calling them "losers."
 * Their voices change when they are about three years old.
 * They do not like people talking about legs and walking, presumably because they want to have legs themselves.
 * They have approximately 17 hearts.
 * Merpeople cannot step (or flop) out of water without suffocating.
 * They can make dolphin noises.
 * Mermando told Mabel that his kind cannot be seen, implying that the merpeople want to keep their existence hidden from humans.
 * The only other Merpeople known are Mermando's family.