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 only 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.

Appearance
The Nymphs both have green-tinted skin and hair that looks like rushing water. One of the Nymphs has long hair and wears a pink bikini. She is also slender and taller than her friend. She wears multiple water bracelets on her arms and leg, and has a pink 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 then her companion. She doesn't have a bellybutton piercing. She has black eyes.

Trivia

 * Adam Muto stated that their looks were based on normal girls.
 * 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 Dr. Princess. She claimed it was how her mother tells a joke.

Quotes
"Tall Nymph: I've got a joke. What did the mermaid see a movie?"

"Tall Nymph: Oh, I messed it up. But it's still funny, right?"

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