Generally regarded as gentle creatures, selkies are seals who can become human, and were supposedly very common in the Orkney Isles. The word "selkie" even means seal, in Orcadian dialect. Even though selkies do not have any fish traits as mermaids do, they are inherently of the sea.
According to surviving folklore, it's not clear how often a selkie could transform, whether it was once a year, every ninth night, or another unspecified time frame. When human, they often dance on the shore or sunbathe on the rocks. According to Orkney Jar, the most important aspect of the selkie myth is their sealskin, which they needed to transform. If the sealskin was stolen or somehow lost while in their human form, the selkie would be unable to return to the sea since they could not transform back into a seal.
Male selkies would often transform into human men and go out to seek unmarried (and sometimes even married) women, who found the selkies attractive. Occasionally, when a woman would go missing, it might be thought that her selkie lover had taken her to live with him in the sea. Selkie women, on the other hand, were often tricked into giving up their sealskins by human men in order to marry them and have their children. Usually, the sealskin would be found by the selkie's children, and in some stories they would follow their mother into the sea, while in others they would stay with their father on land. Even if the selkie's children had the ability to become seals themselves, this was undoubtedly a difficult choice to make.
