The Sealing Key is a key in a shape of a sea horse given to Taro by Sara to allow him to enter Gaito's castle. Aqua Regina used it to seal the Panthalassa Clan and give it to Hippo to guard it but the key was stolen by Sara and she removed the seal.
In the Pure season, the key doesn't really appear, though, in the manga, it is used to cut Michel's whips when he attacks the secundary trio (Coco, Noelle, and Karen). Sara uses the key to abduct Tarou to Gaito's castle. He can breathe underwater with the help of the key.
The key is golden and shaped like a seahorse. Around the head of the seahorse is a golden disk with small decorations on it. Hippo has the power to turn it into a staff, which is mainly used to break stuff like glass tanks or doors. Hippo can turn into the Hippocampus with the key, and, in his human form, transform the key into a staff.