if you want a slight spoiler and an explanation it won't, they mention in the interview of the first volume that at first the human-fish was only going to get mentioned in a dialogue, but Tori Miki proposed to include it and make it so you can't really tell if it's a dream or if it's reality. It's going to be a bit of a recurrent thing, to represent creatures from Natural history in a way that you can't really tell if they are real or not, kinda like the unicorn or the ghosts of chapters 16 and 23. The creatures never have an impact on the story nor to they get close to the main characters, but you can't really tell if they exist or not.