Merfolk are a species accidentally created by the Seelie Fae. They were originally an effort to make humans better and allow them to live underwater. A Fae grew enamored with a citizen of Atlantis and when the city was engulfed by water, he turned her and the city into a human/fish hybrid. Disgusted with the results, the Fae abandoned them soon after.
Their lifespans are shorter than that of a human, by about one-third.
There is not much known about Merfolk and it is believed they are a myth, even among other Supernaturals.