A nixie, also known as a melusine, is a shape-shifting fairy that inhabits fresh-water springs and rivers. Nixies usually present themselves in half human half animal form, the animal half being typically a fish or serpent, like a mermaid. Sometimes nixies have wings as well.
Image from http://www.eaudrey.com/myth/melusine.htm
Nixies can be harmless or harmful depending on the tradition. Some traditions portray nixies somewhat like sirens, luring humans to a watery grave. Other traditions, like the story here, present the nixie as an essentially harmless creature able to coexist and even prosper in the human world, given the right conditions.
I’m sure you wouldn’t think that you’ve ever seen a nixie/ melusine in your day-to-day routine, but I can guarantee you that you have. The lady in the Starbucks logo is a nixie! The tail was more visible in older versions of the logo, but according to the site the image is from, the new logo Starbucks will be coming out with will have the old image on it. To be honest this might already exist, I don’t go to Starbucks often, and when I do I’m not looking at the logo. But interesting no?