Selkie

A Selkie was a seal-like alien from another planet. It could control the weather and disguise itself as a human with a person's skin. Captain Jack Harkness found a Selkie who was the last of its kind on its dying planet. Jack took pity on the creature and took it with him to Earth. On Earth, it became part of a Scottish myth of an enchanted seal that protected its Seal Island home. (TM: Captain Jack and the Selkie)