Selkie

A Selkie was a seal-like alien from another planet that had the ability to control the weather and was able to disguise itself as a human with the use of a person's skin. Captain Jack Harkness found a Selkie that 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 known as part of a Scottish myth of an enchanted seal that protected its Seal island home. (TM: Captain Jack and the Selkie)