Sealight is one of the 3 main Sea Ponies of My Little Pony: Rescue at Midnight Castle
Just like her friends, she also is introduced during the song "Seaponies" she and her friends saved Megan and Applejack by getting them in a clam and putting them in a bubble. Sealight and her Friends after they finished their song and saved the 2 characters, Sealight gave them a small seashell to call them if they needed help again.
Megan couldn't open the gate to Midnight Castle so Megan threw the white seashell into the lake and the seaponies came and helped them get inside the castle by making a raft and sailing them across.
Her Design is Based off another pony, Ember. People called her Seapony Ember. but when the toys came out, their real names were revealed.