Sea Ponies first appeared in the My Little Pony line in Year 2. They had the same head and mane as the traditional ponies, but their bodies were shaped like seahorses and they had fins on both sides.
A second set of Sea Ponies was released in Year 3, with this line including both adult and baby ponies. The adult Sea Ponies came in a clamshell or conch shell stand since they could not stand up on their own. The baby ponies had a floating ring.
The Sea Ponies have some unique qualities that make them prone to damage - since they were meant to be played with in the bathtub, it's common for them to become moldy. The adult ponies also had a metal weight inside to help them balance when floating. These weights are prone to rusting and the rust can damage and discolor the plastic on the outside.