I certainly do believe they are real. I believe many mysteries live beside us, but we are oblivious and not aware. And also मत्स्य कन्याओं are awesome :P
it is clear to see that humans created मत्स्य कन्याओं from sea creatures that are alike with the मत्स्य कन्याओं tail......I mean a मत्स्य कन्याओं and a dolphins tail doesn't really have any difference in looks...so I don't think they exist....Half human half मछली :/