Sadie is a mermaid in the Pickle and Peanut episode "Fishy Biz". Sadie isn't a traditional mermaid. Instead of hair she has a fin and a blowhole on her head, similar to dolphins.
Sadie kept her mermaid identity a secret in order to get a job at an aquarium. She serves as the love interest to Pickle, whom accidentally reveals her secret when he came to profess his love for her. Pickle managed to convince Sadie to go out with in the public, by promising that no one would find out she's a mermaid. They successfully partake on several activities, but during a skydiving, they forgot about the tank, and Sadie was revealed as a mermaid to the world. The owner of the aquarium wanted to capture Sadie, but Pickle managed to stop him. Sadie became the owner of the aquarium, and didn't hide being mermaid again.