Selkie's explained:
Selkies are creatures that are essentially human that can don the skin of a seal and become one. There's not much that sets them apart from humans other than their large pupils and their different culture. This is especially true since if humans wear a Selkie's skin they too become a seal though their eyes will have smaller pupils making them poorly adapted for underwater swimming. Key to the stories of Selkies is that Selkies rarely spend too long out of the water and even after falling in love with land swelling humans they will often put their seal skins back on and swim away back into the ocean at some point without warning. That is unless their human lover hides the seal skin and they are unable to become as seal again. They know all too well that you can't survive simply swimming in human form in the ocean and wont risk it though humans sometimes not knowing the dangers or not believing they are that bad might try to swim out into the sea in human form and die. Selkies are born as seals wearing their magical seal skin and only Selkies can be born from mixed breeding. Selkie males often appear on land in human form to find humans to fall in love with though they are notoriously nasty lovers and prone to toying with or being mean to their partners. Female Selkies often avoid using their human form and venturing onto dry land unless a human shows them great kindness when they are a seal and then they reveal their human identity sometimes asking how they can return the favor. Often very attractive in human form, humans tend to snap up the opportunity to marry them but knowing that they might return to the sea, try to prevent them from doing so. Selkie females are often very maternal and will do anything to avoid being away from their children, often running away into the ocean with their children when they leave. Those that have human children via adoption or marrying into a widowed family will sometimes steal skins from other Selkie children so that their own children will be able to go to the sea with them. In some stories a Selkie's seal skin can be made via a witches magic using a regular seal skin though these are sometimes prone to failure. Selkies sometimes hide their children in the seal skin when they are wearing it making only a slight bulge in the belly of the seal which can be easily missed if they are seen in a glance and it doesn't affect their swimming ability.
Most sources will tell you that Selkies are a Scottish legend and that in terms of mythology the legends are relatively recent, however this is not true. The creature is known by many names and is a worldwide legend dating back thousands of years. The theory is that ancient tribal people found snake skins lying around and came to the correct assumption that snakes shed their skin. However, believing at the time that snakes had magic powers, probably due to worshiping a certain part of the male anatomy at the time, they believed that when the snake shed its skin it was human underneath and could sneak into human tribes pretending to be just a lone outsider. The ancient word for snake is Sak, which evolved into Selk by the time seals were discovered. The word Sak was used for many snake-like creatures including all reptiles and amphibians and generally creatures with a long tail in place of back legs at the time like fish for example so it makes sense it was used for a seal. The Selkie legend is also the origins of mermaids and sirens and despite popular belief it is possible in folklore to have a half-selkie, a creature that has the upper half of a human and the lower half of a seal, though still the seal part is human form in costume. However this is not the product of mixed breeding but the results of a Selkie's seal skin being damaged or deliberately cut up leaving only part of the skin intact. It is also likely that Selkie legends are the origins of the Muleskin fairy tail which itself is the inspiration for the story Cinderella.
