I'm sure I'm not the first person to come up with this idea, but it's been rattling around in my head for a while so here we go.
Every so often when you're talking about fantasy species, you'll get arguments about things like anatomy or what makes sense evolutionarily. One camp is to say "It's fantasy, it doesn't have to follow the rules". That's a valid interpretation and one I subscribe to in a lot of cases. But I also think that in cases where you do want things to 'make sense' from a real world perspective, there's a lot of room for interesting worldbuilding there.
Take for example merfolk. There's plenty of reasons why a half-fish person wouldn't have hair, but consider: what we call a merfolk's hair is actually a complicated array of sensory organs similar to a fish's barbels. This hair, spread out and moving with underwater currents, would give merfolk a sensory advantage under the water when outmaneuvering predators. The actual sense that the barbels impart could vary depending on your needs, but I do like the idea of them being taste organs like they are for fish.
Where things get interesting are in the cultural practices this could spawn:
- A merfolk tying their hair into a bun could indicate perceived safety. It could be a sign of respect for a guest, as if saying, "I am comfortable enough around you to not be constantly on the lookout for danger."
- Similarly, shaking one's head could be a sign of distress or worry, as if they're trying to detect even the faintest hint of danger around them.
- Hair ornaments made from or infused with flavorful materials that could tell you a lot about the individual in question, or be a status symbol that they can afford such ephemeral luxury since any kind of infusion wouldn't last long in the water.
- Merfolk communities near areas with lots of aquatic flora might have a practice of using their barbels as camouflage to escape detection from predators, hiding amongst seaweed and letting their hair add to the illusion.
- Deep ocean merfolk who interact entirely with the world through their barbels rather than sight, describing their environment and people they interact with entirely by the subtle tastes in the water around them.