I don't believe mermaids exist.
Having said that, there are plenty of interesting stories that are fairly hard to explain away (i.e., you might choose to just dismiss them out of
hand, but they are hard to debunk).
The case of The Orford Merman
has always fascinated me. The reason
being that it dates from the 12th Century, but doesn't seem to be related to any existing folklore of the time.
If it was a folktale, it was never recorded anywhere else in the world, let alone the UK. And it completely lacks the usual trappings of a story about
merfolk - the 'merman' wasn't fishlike at all, he didn't sing alluring songs, or comb his hair or beard, and didn't lead anyone to an undersea
kingdom, or reveal the whereabouts of hidden treasure.
Moreover, even though the chronicler who recorded the merman story (Ralph of Coggeshall) was a monk, there is no apparent attempt to impose any kind
of Christian moral on the story, or draw any moral lesson from it. The only religious touch is the observation that the merman didn't behave
appropriately in church, with the implication that he didn't know he was meant to. And that's halfway through the story and Ralph doesn't make any
great deal out of the incident.
In fact, there's a sort of ordinary and pointless quality to the story. (Spoiler: Fishermen catch a merman, bring him to land, can't figure out what
to do with him, let him go swimming in the sea, and he escapes. Then, and this is the odd bit, he voluntarily comes back, stays for a few weeks, and
then escapes again and disappears).
As I say, I don't believe it for one minute, but thought I'd give it an airing on this thread, for what it's worth.
The rest of the blog I've linked to in the above has other merfolk tales from antiquity, if you're in the mood for that sort of thing.