Selkies as a fantasy species have enamored me ever since I was a little kid. As the quote I use in the homebrew suggests, I first heard of Selkies through the Magic Tree House book series. Those were definitely kids' books, but if you ask me that series' titles were great at being kids' books! Summer of the Sea Serpent was among the early titles to introduce more expressly magical elements in a relatively short-lived, but still enjoyable direction for the series. This book marked Kathleen's introduction, and Kathleen was the first Selkie I ever met.
What are Selkies? Outside of The Magic Tree House, that is? Selkies come from Scottish and Irish folklore. They are creatures that can transform from seals into a (usually female) humanoid form. Probably the most famous Selkie trope in the popular conscious is that of a fisherman or other man stealing a Selkie's sealskin in order to obligate her into marrying him. I reference this in the second page sidebar; ultimately, I decided to write the mechanics such that this scenario is not possible. I don't want to write mechanics that would cause players to lose agency, and I think Selkies have the potential to be a lot more interesting than prizes for a man—not to speak too on the nose about this, but it is a relevant motive for sidestepping that narrative.
Next question: why do I want to be able to include Selkies in a TTRPG setting? Well—just because I think they're cool, really! Sometimes that's all the reason one can have. A wise designer once told me that in homebrew, self-satisfaction is a valid and important motive. I guess then the qusetion becomes why do I like Selkies? There's something really fascinating about the dichotomy of Selkie life.
Sometimes I compare them to Mermaids to emphasize the differences: Mermaids spend their whole lives living in the water, and their humanoid shape and fish tails are always part of who they are. Selkies, meanwhile, don't exist as a physical in-between; rather, they swap entirely between spheres of existence, going from ocean-dwelling seal to land-walking humanoid and vice versa. That dual-lifestyle is incredibly interesting! I hope that came through in the lore and mechanics of the Selkie I created here.