Wavedancer is one of the Sea Ponies from My Little Pony: Rescue at Midnight Castle.
Wavedancer, along with her friends (Or sisters) Sealight and Seawinkle, Save Megan and Applejack from the sea by Singing their only song "Seaponies (also called Call upon the Seaponies and The Sea Ponies Song)" Wavedancer and her sisters (or friends) then put Megan and AJ in a bubble and lets them out when the song was done. The same 3 Seaponies return to help Megan and the ponies get inside the Midnight Castle. the same 3 also appear during the credits.
Her design is like a Seahorse version of Firefly. people call her Seapony Firefly but that got later resolved when her name was revealed via toys.