Water Nymphs

Nymphs are water spirits from traditional folklore, in this case depicted as having moving bodies of water for hair and wearing swimsuits.

The Nymphs make their first appearance in the episode "Power Animal." They're playing in the pond near the Tree Fort telling terrible jokes to each other. They later attend a party in the Cloud Kingdom.

The Nymphs are known for their lack of being able to tell a good joke without messing it up.

They make there third appearance in "Thanks for the Crabapples, Giuseppe".

Appearance
The Nymphs both have green-tinted skin and hair that looks like rushing water. Denise has long hair and wears a pink bikini. She is also skinnier and taller than her friend. She wears multiple water bracelets on her arms and leg, and has a gold bellybutton piercing. She also has pink lipstick while her companion has none. She has white eyes.

The other Nymph has short hair, similar to Princess Bubblegum's at the end of the episode "Death in Bloom," and a blue bikini. She is slightly curvier, shorter and has bluer skin than her companion. She has no bellybutton piercing. She has black eyes.

Trivia

 * Adam Muto stated that their looks were based on normal girls.
 * Denise (the taller nymph) is voiced by Hynden Walch, the same person as Princess Bubblegum. The shorter nymph is voiced by Niki Yang, the same person as Lady Rainicorn and BMO.
 * The forgotten joke is also part of the stand up routine (at 3:03) of comedian Melinda Hill who does the voice of Doctor Princess. She claims it is how her mother tells a joke.
 * The Nymphs appear as save points as well as in the Cloud Kingdom in the video game Adventure Time: Hey Ice King! Why'd you steal our garbage?!!
 * According to the comics, Ice Queen was originally a Water Nymph.
 * The long-haired nymph appears again in "Wake Up," where her name is revealed to be Denise.

Quotes
"Denise: I've got a joke. What did the mermaid see a movie? Other Nymph: Wh-what? Denise: Oh, I messed it up. But it's still funny, right?"

"Denise: You blacked out after laughing so hard at your own jokes. Other Nymph: You are off the hook! Denise: So we brought you to the Cloud Kingdom to party with the Party God."